I used the search feature for my problem but didn't find a real good answer. I have a 2001 f150 extended cab 2 whl dr with 98,000 miles. I have a knock in gear in the engine and upon slight acceleration. I took the serpentine belt off, started it, put it in gear, and the noise disappeared. Put the belt back on the knock reappears. I would have bet it was the tensioner pulley , replaced it same knock. Anyone else experienced this and if so what is the fix? Thanks

 

Check your water pump or your a/c clutch.

 

Could also be the fan clutch.

 

Fire it up and very carefully put a stethiscope on each component. Kinda like cave man tools in a digital era, but it works pretty good to isolate noises. Good luck.