I have a 99 3500 auto with 160,000. When you step on the throttle hard I get a noise from either the trans or the engine area that sounds like grinding a piece of plastic on a bench grinder.(best way to describe it) It seems to be when the engine torques. Checked all obvious-torque converter housing, fan, belt, etc. Can't find a thing. Any ideas? thanks.

 

Double check the exhaust manifold for looseness or possibly a crack.