It could be the wrong belt or a worn tensioner pulley spring.
I recently had a similar issue on my 99 Burb....squealing at cold start-ups. The whole thing looked good, stayed in place, and charged just fine...but, it would squeal when it was cold. It had been doing it for years but, it was getting worse. I figured it was a worn tensioner pulley assembly. So while replacing the belt tensioner, I noticed that when I released it...the pulley went clear out to the limit of its travel. It seemed as if the belt was slightly too long and the tensioner could not get the belt tight enough. This struck me as "odd", so I did some investigation, and discovered there are two different amp alternators available that year. Each alternator has a different size pulley, and thus...a different length belt. The length difference was less than an inch, (hardly noticeable) but, after installing the correct belt...it became very obvious that I had been running the wrong belt for quite some time. Now the tensioner sits in the middle of its travel, and the belt is noticeably tighter, (now that the tensioner can apply full spring pressure). Squeal gone!
I'm not saying you have this issue but, its an example of just how sensitive the serpentine system can be. You might want to cross-check the belt number to make sure you have the correct one installed. Sometimes a counter guy will simply use a belt sizing tool and grab the closest match he can find...without really checking the numbers. If a longer belt has been installed, it could be that your tensioner is sitting on the limit of its stroke, and not applying enough tension.
