So I have a '96 blazer. Recently I was sitting in it, about to go to work and i hear a loud hissing sound as though a bunch of gas was escaping. Almost like an airbrake on a semi. I was running the AC and immediately after the hissing noise started, my AC started blowing hot. I asked around and was told it was either the compressor or the hoses. I figure no big deal, it is getting ready to cool off. Well now there is a fairly loud humming noise coming from under the hood. It goes away temporarily when the engine is reved but as soon as the rpm starts droping it comes back.

Any ideas what I am hearing?

 

have you looked for any leaks anywhere under the hood? have you tried to disconnect your belt and start the truck to see if the noise stops? leave the A/C off too

 

I have looked for leaks, but there were no hole that stuck out.

I will try to take the belt off and see what happens.

 

I would look for bearings going out in your serpentine system. If the A/C was blowing hot this could be an indication that the belt is slipping when the A/C clutch engages...posibly seized A/C compressor?