Going to try and make this as legible as possible with a splitting migraine.

The start:
2004 Buick LeSabre. 122,000 miles.
Has FE5 suspension, some new parts, some parts off a low mile 05 GXP from Morad.

I'm going around and around with this car, and I can't fix the vibrations I'm getting from the front end. Every single part that can be replaced (with a couple exceptions) has been replaced. And not one single thing has been able to get rid of this. In fact, some have made it worse.

Feels like: tires out of round, but only at times, getting some brake pulsation, but only at times. It feels very planted, it doesn't wander and it tracks straight. But honestly, it feels like a wheel is gonna fall off!

I've done both bearings, 3 sets of CV axles (stockers, GSP aftermarkets, then Dorman), New rotors twice, new outer tie rods, new upper strut mounts, full set of new tires that were road force balanced, old tires were balanced at 3 different shops and I still have the same result with the new tires.

You can feel it going down the road, light corners make it worse, heavy corners actually make it better (gotta love the FE5!.. handles like a slot car).

I can't pin point down where this exactly started. I've done pieces of the FE5 in stages. Although changing out suspension parts shouldn't seem to affect this. But the last step, the anti roll bars, seemed to really amplify the whole thing.

One constant, I get a clunk from the driver side when shifting reverse to forward, or the opposite. Every time. None of the axles have taken away that noise, though it would seem like a CV joint popping. I can have my foot completely on the brakes, and it will still pop when changing gears.

I've had 3 shops look at it, and they seem to think its fine. But they don't push the car like I do. I put the FE5 in there for a reason. I drive twisty mountain roads to work everyday, and I love tossing it around. I get no tire squeal, no wander, no instability (no it doesn't have the stabilitrack system). I can point it into a corner, and it just goes. Again, the harder I push it, the more the problem goes away.

I just get sick of hearing "well, its a 6 year old car, its not gonna drive like a new car... its not a Mustang, you can't expect it to corner like one". Even though the problem really isn't in the cornering ability at all. They take one look at it, listen to what I've done, and attribute any harshness or vibrations to "well, its gonna ride harder with that setup". Well, yeah, but it shouldn't vibrate my teeth out going down a fresh paved road at 30mph, either!

I feel like I'm loosing my mind, because I'll throw parts at it until the end of time, and it will never be right. It was fine when I first bought it. That was back in November last year. The main difference is that back then, I was driving 30 miles a day to work and back. Now its more than double that. When I started this new job, I started to see the changes. At first I thought it was maybe the roads, but its not. I've taken my Cruiser down the same road, my dads caravan down the same roads, and yes there are spots that "washboard", but its not like that on the whole road.

I'm not sure where else to go with this. I could easily swap FE1 back in, but honestly, I don't think the problem is with the stiffer suspension.
If I had $800 lying around for a pair of new GM axles, I'd buy em, just for piece of mind, but I've already been through 2 sets of aftermarkets, and neither has done squat.

I haven't done much of anything with the rear end, except the FE5 specific parts. But the problem is definitely up front, even slight braking sometimes gets a pulsation, and the clunk from the left is obviously in the front, being drivetrain related.

So... whats next? New trans, new diff? I'm just tired, and broke from throwing parts at this car, plus not having any shops around here that cater to "non-stock" problems. Every one of them wants to call foul on my FE5. Some don't even want to touch it (ie, dealer.. "You put parts on a car that wasnt meant to have them, of course you're going to have problems". (Service advisor, and head tech, after listening to my problem).

Anyone have ideas? Tests? A shop somewhere in central NY that has some idea what they are doing with a car that knows more than "well, try this. no, okay. try this then. Nope, that aint it. Lets just keep spending money and throwing parts on it, eventually it will go away..."
A true mechanic is a dying breed. Everyone working on cars now is just a parts changer, and can't diagnose their way out of a paper bag.

Help. The more I drive this car, the more I hate driving it, mainly because I know there is so much potential there. If I hadn't spent all this money tracking down this problem, it would have a top swap right now! Its almost like a nagging migraine headache that wont go away (!!!) I love the car, but this whole mess is making me start to hate it.

I'm seriously considering taking time off, going to see Jack Cotton, and having Jack and his son look at it, because honestly, Im sick of parts changers, sick of people who don't have any idea how to work on a car besides throw parts at it, and Jack is really the only place I know of that has treated me well in the past, and could care less if you throw a bunch of non-stock crap together. His place is one of the few left I know of that will "diagnose" a car, not just replace parts until the customer'* wallet is empty. Problem is, Jack is 4 hours from me. But, if it has to go to that point, then so be it. He'* done great work on my Turbo Regal, and I'd trust him with anything from a Cavalier to a ZR1, knowing its in good hands. If thats what it takes to get this car right, then thats what it takes. But I'd rather do as much as I can on my own before I resort to that 4 hour ride that my car might not even make.

Couple of dumb questions, but ones that were not addressed none the less.

Tires/rims? Have you rotated them front to back to see if the problem moves?

Problem getting worse with the FE5 stabilizer bars. That would make sense if the issue was specific to a certain side. One wheel would be shaking up and down, and the stabilizer bar would be sending that to the other wheel. That may or may not be a clue here.

Can we get a list of parts replaced, along with points of interest when it started happening, what parts made it better, what made it worse, etc?

That clunk on the drivers side is rather curious also, especially after replacing cv joints.

Oh, now that I look at it, I see the tires and balancing, and I see most of the parts replaced.

Have you check motor and transmission mounts by chance?

The motor doesn't seem to move much at all. Really, the rubber parts on this car (bushings, control arms, etc) seem to be in pretty good shape considering being 6-7 years old, and in upstate NY.
I actually had that thought in mind at one point. Took my ratchet strap (its a sort of motor tie down made specifically for my 87 Regal) and hooked it in, to see if holding the motor back would make any difference, to see if the mounts were loose, but it didn't.
I actually wanted to make my own motor tie down, but I haven't had the time or really been able to find a short enough ratchet strap to use as a starting point. I know the Buick motor mounts tend to be a bit "cushy" as is with most Buick parts, so I wanted to tighten the mounts up a bit and take some of the play out of it.
Never have gotten any further into messing with it, other than that initial test.

The problem is, the suspension itself seems tight. I haven't replaced the LCAs, but again, the car tracks very very well. Its just like having a tire out of whack. There is no slop in the steering, no loose side to side play. You point the car where you want it to go, and it goes.

Its a very rhythmic vibration. Like a nail in a tire. When it happens. Sometimes its smooth, not even there. Other times it feels like the wheel is ready to fall off. Sometimes the brakes are fine, sometimes they shake the fillings in my mouth. Some hard stops are smooth, clean, no vibration at all. Some feel like I'm stopping on cobblestones.

Actually seems to be happening more often than not now. With the 4 new tires, and the sway bars installed, more and more of my drive (no matter the road surface) seems to be full of vibrations. Whatever it is, is getting worse. Or the new tires and sway bar are amplifying what was already there.

the mounting location for the control arms to the engine cradle might be ovalized.

But that would cause a change in cornering behavior, since it changes alignment. I don't think whatever this problem is, has anything to do with alignment. Its more of a drivetrain vibration than a suspension vibration. Suddenly accelerating or hitting the brakes doesn't change the attitude of the car. It tracks straight, no matter what the vibration is doing at that point.

How can I check for play in the motor/trans mounts? Its something to think about, but then again, hard acceleration or braking doesn't have any repeatable effect on this vibration.

Anyone have more thoughts on this?

I'm going to be under there later today swapping out the oil pressure sensor I smashed trying to get one of the old axles out.

What else would cause this snapping sound out of the driver side? It has done this with 3 different axles now, none changing the sound.
I can put the brakes full on, and it will still pop when going from forward to reverse, then again going back.

Is the trans going south? It shifts fine, but with all the other parts changed, makes me wonder if the diff is going out. It takes sharp low speed corners in a parking lot ok though.

ok im a technician by trade so let me try to help you out the best i can. im gonna assume all the suspension is tight. the popping or snapping when going from reverse to drive is almost always a loose or improperly torqued axle nut. not saying it couldnt be something else but thats what it usually is. as far as the shaking i had this problem on the highway bad, bad, bad. i did find that i have a tire that isnt completely true but regardless i removed the rotors and cleaned the hub surface of all rust, back of the rotor mounting surface, front of the rotor mounting surface, back of the wheel mounting surface, rebalanced tires and torqued the lug nuts to 100 ft lbs. this fixed my problem that i have been experiencing for almost 2 years. i never thought mounting surface cleanliness and wheel torque mattered that much but obviously they do. this was several weeks ago and its still good. if you havent done this already try it you have nothing to lose. also make sure tire pressure is at spec that may cause a basketball effect as well. these cars are real sensitive. i share your frustration cause i went through it too.

The axle nut is torqued to spec. I have the FSM I'm going by, and a calibrated torque wrench. And its on its 3rd axle. On each side! At some point, through changing 3 axles, and using a new nut every time, I gotta say I think I have that covered.
And I agree with the mating surfaces idea. Which is why I did new rotors when I changed the hubs. I had a PS bearing hub go out, and I swapped both sides out, along with new rotors, so that I knew the connecting surfaces were clean and true, and there wouldn't be any rust build up. The wheels I actually did take to a shop to get balanced, and they actually cleaned them up for me, scrubbed any trace of crud off the back sides of them, so they would sit flush against the rotor hat.

The pop from the left front, it doesn't instantly pop. And if I gently ease into which ever direction I'm going, it won't pop. But its almost like it winds up, then lets loose. Almost like a strut spring on a bad mount that pops when the spring gets wound up. Its not an instant pop, it takes it a second to wind up first.

I also noticed, going for a drive today, the brake shudder is worst at about 30-40 mph. If I hit the brakes hard above or below that point, it is fine. I believe I am getting shudder in the driveline at the same speed if I accelerate within those speeds as well, but its tougher to tell.
I drive the same road to work and back, 5 days a week, so its easy to tell when something is different, or behaves better or worse. I could drive the road in my sleep. So I tend to notice all the little things. Like, it does almost seem worse on right hand turns than left turns, no matter what speed. Maybe its the road, maybe not. But it seems predictably worse taking right handers. Even on the same exact corner, but going the opposite direction.

I am curious, now that you are able to predict it. If you pop the car in neutral, before slowing in a manner that would replicate the results, does it still do it?