After doing some other work on my car and noticing a hose clamp rubbing against I belt, I thought that was it. No luck though.

I did however notice that my exhaust pipe is rather loose and does in fact vibrate up and down. I'll have to look at it once im under it again. But I do believe the exhaust pipe is probably making this noise.

I have a 98 SLE it has a noise around the motor, but i noticed the more i drive it it dosn't get worse so i had my diesel mech, at work check it out he said that it'* the flex plate. cause the flex plate is always in motion at drive or park. it'* louder then a lifter quiter than a rod. yours may be broke or just loose a mech can remove the insprcetion sheild under the tranny to check it pretty easy to do... Good Luck