Well, my exhaust fell off the other day. All I can think is that the noise I was hearing only occured in situations when the car had a torsional load on the body- pulling into/out of parking lots that weren't even with the roadway- and that what I was hearing was the exhaust cracking apart due to corrosion. Now that it is welded back together, I haven't noticed the noise, although it's hard to focus on listening for a noise when you're watching traffic.