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.

Noise With Bonnieville
 

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's the flex plate. cause the flex plate is always in motion at drive or park. it's 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