I have complained about this to 5 different mechanics for my 2001 Deville and no one could ever figure it out. It finally got real bad and at my most recent oil change the mechanic showed me the right rear LATERAL ARM bolt is broken off the control arm. It is only noisy when I go over bumps. It rides completely smooth when on the highway. No steering wheel shake or anything else. I'll let you know what it cost to fix since I am not a DIY guy.

The rattle is cause by debris in the mufflers most likely caused by the catalytic converter it sounds like a suspension issue when hitting a bump but is not. The easiest way to check this is wrap a thick towel around a hammer and tap on the bottom of each of the duel mufflers.

Thank God for forums
 

I own a 2001 Cadillac DeVille that I’m truly enjoying. I recently heard a rattle coming from what sounds like the left side in rear as I went over bumps. I’ve had A TON of work done on this car since the purchase of it in January which included the entire engine being removed and getting the common head gasket issue fixed before it even thought about creating a problem to an entire new Strutmasters air suspension conversion put on it. Car was riding amazing until the annoying noise started coming and I thought it was something loose in the trunk. Tried checking spare, under panels, shaking rear of car, until one day it started driving me batty enough to do a Google search for such a broad question, and bam, up pops this forum. I’m going through all the replies and suggestions and going to get with my mechanic to discuss the issues and what people in the forum are saying it could be. I’ll update later!!!