noise when A/C is on

[ccarr003]

Hello everyone. First time posting. Need some help and advise before spending a bunch of money to fix may problem. Started my truck this morning and it ran fine. Drove about 20 miles to get to where I was going. The next time I turned on the truck I noticed that the vehicle was vibrating more than usual while ideling. I pulled out and this horrenous fast knocking came from the hood. I stopped and turned the A/C off. The truck didn't vibrate any more and sound great. I accelerated when in idel with the A/C off and it was fine. When I turned on the A/C and accelerated while in idel, the load noise came again. I popped the hood trying to located where the noise was coming from. Had the wife step on the gas while doing this. I had her accelerate while in idel with both the A/C on and off and the noise only came when the A/C was on. The noise appeared to come from in front of the engine. I saw no obstructions in the fan. It sound like a fast paced metalic knocking but I still wasn't sure where the noise was coming from. I also noticed a loud hissing noise going on and off while driving with the A/C on. If anyone has any idea what I'm talking about, please reply. Thanks for reading.

PS - 2001 Chevrolet Suburban 8cly 5.3

[chuck's]

With the a/c on can you observe the belt and tensioner for the compressor?It is quite possible that the compressor or the tensoiner is failing.I have only come across one in which the tensioner was vibrating violently which was then replaced with the compressor.I hope this will help you.
chuck's