Mystery Grind in Transmission, Engine, Where?

I recently purchased a 1994 ETC with about 130k on it for 1800. The car looks wonderful but has a few flaws, one being when put into gear after its sat a bit (few hours) it will make a grinding noise up front for a brief period of time then quiet up. If its shut off then started up just after it wont make a peep, only after its sat a bit will it. Ive noticed that if I have my foot resting on the brake pedal I can feel it vibrating with the grind BUT it will grind without my foot on the pedal either way. Anyone else experience this? Ive been told in another thread that it may be the a/c clutch but I want to make sure. If the clutch pack was slipping would it make this noise as well? Tends to sound like grinding the gears on a manual but slightly softer.