2003 Chevy Impala squeaking blet

It could be:
bad bearing, misaligned pulley, loose belt (stretched), or bad belt tensioner.
Here something you can do to find out what's going on: spray the serpentine belt with a spray bottle while the engine is running. If the noise stops (or decreases a lot) then comes back loud you're probably dealing with a misalignment problem. If the noise increases after you spray the belt, your belt is probably loose (bad tensioner or stretched belt).
This test will be inconclusive though if your problem is a bad bearing.

Another trick to find out if your problem is due to misalignment is to remove the serpentine belt and flip it around so that it runs in the opposite direction.
If the noise doesn't come back when your turn the car on (or not as loud), you've confirmed a misalignment problem. If the noise remains unchanged, misalignement is NOT your problem.

Hope this helps