wierd "Diesel-like" knocking

ive heard this sound pretty much all summer since i started using the ac. i always thought it was the ac compressor and didnt give it much thought as it was always the same sound at idle when i was out of the car. a steady knock like an idling diesel motor. i was at the drive thru of a fast food place yesterday and with the window down i could hear the same sound, so i turned off the ac and it quit, i was thinkin ok, it is the ac compressor. then it also dawned on me that the compressor cycles on and off as to the need of it. btw the car knocks with the trans in neutral (5 speed) and parking brake on. so i turned the ac back on and the knock came back. i pushed the clutch in to move forward and it stopped. im stumped, i got to work and opened the hood and couldnt really locate the exact location of the knock. not the ac compressor for sure. the car is a 93 with 172000 miles and still runs,drives and idles great. so i would think it has something to do with a load on the engine, albeit however small. i havent noticed the knock at speed as wind noise/etc is to loud. anyone experience this or have any ideas?