If you're sure of that then I would check the installation. The only time I've seen squeel and not be able to pinpoint it is when thisgs weren't lined up right. Such as the tensioner sitting out just a tad; it looks good but its not 100% parallel to the belt so the belt squeels while going over it

Usually when a belt squeals, it is because the belt is slipping on a pulley that is too tight, or it is slipping on a pulley drawing power when the belt is too loose. If the tensioner spring is new, belt tension should be OK. You can buy a little tension gauge called a "Klickit" for about $15 to make sure the tension is up to spec. If you have a bad pulley that is causing the belt to slip, you can look for clues as to which it is. Rubber dust builds up around the slip point and the bad pulley will get hot from the friction.

Spraying a pulley with WD-40 and having things quiet down would make you think the pulley is bad. And maybe so, but, maybe the WD-40 is fogging onto the drive side of the belt and quieting it by letting it slip without noise until the oil film burns off. After it has been squealing for a few minutes, shut it off and feel all the pulleys to see if one is really hot. That should tell you where the problem is.

If you are sure that the lubricant is not getting on the belt but is just quieting a screeching bearing on a pulley - then the pulley is bad. New or not - it'* bad.

I took the belt off today and everything turned smoothly and freely, the waterpump seemd to be a bit heavy, but it still spun smoothly.

I had the same symptoms on one of my cars - squealing only at low rpm and having it go away temporarily whenever I put something on it.
It was driving me crazy since belt looked good and I had recently installed new waterpump, alternator bearings, tensioner, and idler pulley.
I put a Gatorback on the car and squeal completely went away.
After driving with the new belt for a while, I noticed the paint on the new idler pulley was not wearing off evenly and the belt seemed to faintly show a little more wear on one edge. Looking closer, I noticed that the belt surface of the pulley was slightly cone shaped.
This experience made me a firm believer in the Gatorback and its ability to compensate for unevenness.

wow, I was just goign to ask about gatorback belts! Thanks for mentioning that! I was lookign at my tensioner puller, and the dark band left from the belt is totally not even, so maybe that is a cause, id. I will definatley be getting a gatorback belt, and seeing if it works!

My wifes '98 had a squeeling belt over and over again, I sprayed the tensioner and even replaced it just as you did, plus I had the alternator rebuilt. No matter what I did, it squeaked until I found the right belt. I bought a GM, I bought a NAPA, and I had a stocker, all squealed. Go out and buy a Goodyear GatorBack belt. Its a completely different design and it says "the quit belt" right on the package.This resolved my problem! This belt is guaranteed for life and guaranteed not to squeal.

All of my problems started when I lost a water pump, the water will ride into just about everything with a bearing plus saturate the belt. If you sprayed your belt with WD40 or even belt dressing, take it out to the woods, say a prayer, and bury it

Hey, I have just been going through the same problem, I replaced both tensioners, and all my pulleys. Still, I got a squeal. After spending all that money, and guy named Crhis who works at the parts counter of my local Pontiac dealership told me to buy some silicone, and apply some on my smooth pulleys. I did it, and now the junker sound is gone, lol