Well, I've had my trouble-free 1999 Deville for a year now, and this is my first big diagnostic problem I've had to deal with, so I'm pretty happy :) but...
Upon hard acceleration (like on-ramp merging) I hear a groan type noise from the rear suspension area but for only a second then its gone. It only seems to
do this when its cold, and even then nothing more than a few times each morning. I thought someone might have an idea before I try to find this phantom noise, since its tough to diagnose a noise that only occurs a few times... :banghead:
Any help is much appreciated! :hmm:

Sure it is not the exhaust pipe grounding against the underbody somewhere??

The exhaust pipe "grows" quite a bit as it warms up from cold so it could be misaligned and touching somewhere and causing the groan as the acceleration moves it against the body.

If the exhaust was ever hit by debris or grounded backing into a curb stone or something it could easily be misaligned.

Try brake torquing the engine against the transmission in drive and reverse when it is cold and see if the exhaust is causing the problem. Hold the brakes firmly and put the trans in gear and load the engine against the trans to load up the system and move the engine on the mounts as if it were accelerating.

A combination of worn motor mounts and a slightly bent crossbar under the exhaust system can cause this. There is a crossbar that runs under the exhaust system near the cat converter. When you accelerate, the front of the engine rotates upward and the rear downward, if there is not enough clearance between the exhaust and the crossbar, the pipe will "ground out" against the crossbar. We have run into this alot, especially after a muffler shop installs an after market converter.

You guys were exactly right. I put the Deville up on the lift today at work, and lo and behold, there was a bent support bar underneath the exhaust pipe with only about 1/4in clearance so combined with my factory original worn motor mounts, its rubbing a nice shiny spot into the pipe when I get on the throttle... so I removed it...for the time being at least. :helpless: