Just installed a new alternator and drive belt on my 2001 Toyota Camry after I noticed the battery light on the instrument panel. Immediately it started squealing and then it went away, and now it only squeals when I turn on the AC. When the AC is turned on it has a slight squeal every 10 seconds or so. Everything that I’ve looked up online is saying that the AC compressor clutch is going out but I never had any issues before I replaced the alternator and the belt.
You said you replaced the belt along with the alternator. My guess is that when you replaced the old belt it has been on there a while and it never showed wear on the AC compressor therefore there was no squeal. I would replace the AC compressor. You did not specify when the drive belt was replaced last, but I am guessing it has been on there a while.
The belt is probably squealing when the compressor clutch cycles. Could be the tensioner is weak after 20 years.
Yes that sounds more like it could be it. Do you know what I might need to replace?
According to this video you don't actually have a tensioner, the belt tension is adjusted manually by pivoting the alternator. There is no doubt a specification for belt tension that you can check with a belt tension tester. My experience with manually-adjusted serpentine belts is they need to be tighter than you might think to prevent squealing - but not so tight that bearings are damaged in alternator etc.
https://www.youtube.com/watch?v=kG2HVhm7mfE
When I replaced my water pump, it squealed like a banshee for a while. I re-did the tension and as the belt seated in, the noise went away.
