Guys - here is what caused my "cowbell".
http://www.m3post.com/forums/showthread.php?p=9505484
There's no way I'd have found this if I (we, actually) hadn't had the car on a lift and been able to shake the hell out of everthing under the car, and then been able to visually inspect it all to track it down.
And, it could very well be that there are other exhaust clamps, hangers, etc., that could cause the same thing, some of them perhaps hidden even better than mine was.