Kenbossy, you're right. If it was a rod knock, when you ground out each cylinder, the knock should have stopped or drastically decreased. Does it do it only at idle, or when the engine is under a load? Does the knock sound like a double knock? If it's a double knock, and you ground out the cylinder, and it still persists, you're looking at a wrist pin problem. If it's not a double knock, and it still persists when you ground out each cylinder, then you may very well be looking at a main bearing knock. Main bearing knock can not be isolated to a particular cylinder. The sound can also vary in intensity and disappear at times depending in engine load. I agree with everyone else when I say that I would just have to hear it for myself. I hope I was of some help. Good luck

 

The spark plug wire trick works sometimes but not always. I have seen some 3.8's with bad rod bearings do this. I would also think a wrist pin could be the problem. are you sure that it;s internal? does it change with RPM? do yurself a favor and remove the belt and run the car to make sure that it;s not an acsesory doing it. you can also take a long peice of hose and use it as a stethiscope to help pinpoint the noise. Good luck