Engine Knock

I have a strange problem with my truck. It is a 91 GMC Sonoma 4.3 V6 TBI. When my truck is idling it has a knocking type noise that is only heard when idling. As soon as you touch the gas pedal it goes away. The engine has not lost any oil pressure and does not burn oil, and just had a recent tuneup. I bought this truck off a old guy and it has been very well maintained with only 60,000 miles on it. If this motor needs an overhaul this might be the last GM i buy. I just sold a S-10 with 40,000 miles because it was doing this, now i have another bad one.

Try this, take off the serpentine belt, and start it up. Does it still make the noise? Don't do this too long however, or you'll overheat it fast, as your water pump is not turning.