Hmmm, that's a new one. I'll look into that.

 

A good friend of mine is a GM master Tech, and major gear head. My wife's car, a 2000 GP GTP Supercharged 3.8, made a horrible clanging sound only at idle. The car is heavily modified to run 13s on stock boost. After listening to it, my friend said it sounded like a balance shaft. He said it was common for to get play in the rear bearing (transmission end of block). Any rpm off idle stopped the sound. That was two years ago, and it still does it to this day. We do now use a 10-30w Synthetic, which helps with the noise a little.

 

just my .02 cents but in the 5 months of this thread has the noise gotten any worse , louder, grindier ( if thats a word and it should be) I was thinking for the area of which you discribed maybe pilot shaft bushing or bearing at back of crank shaft on transmission end, but that would be on the drivers side of the car, and you described sound is from back of engine on the other end of the engine, so maybe the belt tensioner and frayed belt are the culprits

 

The sound seems to have subsided, it pops up every now and then but has not gotten any worse. Now I have some suspension issues I need to address.