2000 Sunfire Possible Alternator Problem

I have had a problem with my 2000 Pontiac Sunfire. For the longest time, it has made a chainsaw noise. I'm not very savvy on cars, but my dad knows a little and he looked at it. He used a stethoscope with a long stick on the end of it to find out where the noise was coming from. The noise is coming from the alternator, or at least, something connected to it. At first, I thought it was some kind of belt because I've been told that cars make squealing noises when a belt is bad. This noise is getting progressively worse, too. When I turn my car on, the chainsaw noise immediatly starts, very loudly. It doesn't get quieter, and it's always there. Before, it would slowly go away, and sometimes, it wouldn't make a noise at all, but now, it's always there.

Also, when I turn on my air conditioning or heat, the noise stops immediatly. It doesn't come back until I turn the AC or heat off.

My dad seems to think either A) The alternator is bad or be ) The clutch on the compressor (I have no clue what this means).

One other thing I will mention is when I put my foot on the gas pedal and accelerate, my car makes a hissing noise.

Can anybody help me? I am going to take it into a shop to get it fixed but I want to have some idea of what it could be so I know what to expect cost wise.

Also, to replace an alternator, how much would that cost? Or whatever you think is wrong with it, how much do you think it would cost to fix?

Thank you sooo much!