A/C problem

[KW900L]

I have a 1994 GMC K1500 extended cab 4WD with a 5.7L engine ,A/T,A/C.The problem I'm having is a knocking coming from under the dash board from the upper "box" that sits above the heater core.It started a few days ago when I used the A/C to take my cat to the vet.I rarely use it.It made a grinding noise,the compressor,so I shut it off and from then on I get the knocking sound. It will quit for a minute when I turn the fan on then start back up.It also does it when I turn he engine off and will continue for a couple of minutes after.It also does it when I turn just the key on like I'm going to start the truck. I pulled he glove box out to see if I could locate the noise and like I said it is coming from the 'box"above the heater core.

The grinding noise was a mounting bolt that was loose and lost the the retaining nut and got into the compressor pulley.Any thoughts on this would be greatly appreciated.Thanks.

Tony Rebustillo