Ive never experienced any sort of sound you are describing that ended up being a transmission/suspension/engine problem. Although its tough to describe a noise in writing, and if you are positive its not your heatshield or the cat, then it might just be an exhaust leak. Is it really loud, or do you just hear it when you are driving next to a wall or another car? Does it sound like marbles in a tin can?

An exhaust leak is really easy to pinpoint. Put in on a lift and you could actually see exhaust coming out of the leak, or put your hand next to it and feel it, or put a mirror to it and see if it fogs up. To diagnose a bad cat, just hit it with a wrench and you'll hear that the insides of the cat are broken and sounds like marbles in a tin can, same with heatshield.

You really need to put it on a lift and really check shit out under there. Good luck.