2001 GA recently I hear a loud screaching noise from under the hood passenger side. Took to my mechanic and he changed the serpentine belt the old one wasn't any good.

Left the mechanic with no noise. Well after I leave work it sounds again. I turn off my Heater and the sound goes away. Now my heater is off but the noise still sounds sometimes. Any advice would be helpful ...thanks

i forgot to mention that the last week or so my heater wouldn't turn on right away. it would take about 5 minutes or so then turn on. Maybe these were signs of a failing AC compressor?

Yea i have a 94 grand am and it does the same thing with the heater. I changed the surpetene belt to and it still sounds like that...

I would check the tension on the belts, make sure it isn't too tight or too loose, either way would cause the noise. When you put the heater on, it's causing some strain on the engine which in turn causes the belts to screech.