1996 Nissan Sentra Aircon Squeaks

Air Conditioning problem
1996 Nissan Sentra Manual transmission 96000 miles

I have this problem for years now. When I start my car, everything's fine. No sound, very quiet engine. But when I turn on my aircon, there is a very loud sound. It's like squeaking. But when the engine's hot enough, the sound will disappear. But the next day, when you start the engine and turn on the AC, the sound will be there again and disappear again after around 10 minutes. When I turn off my aircon, there will be no sound at all though. I have changed my compressor, had my belts tightened also. What seems to be my car's problem? Thank you so much!