When my 1992 745t, 240k miles, is stopped in gear, I hear a noise that I had thought was a bearing going out somewhere in the belt drive system, kind of a grating squeaking sound. Now I've isolated it to the undercarriage, in the general vicinity of the catalytic converter. It goes away with any increase in RPM, and sometimes even by turning the AC either on or off. My mechanic can't replicate it (of course). Is my tranny about to seize, or do I just have a worn hanger somewhere in the exhaust system?

(Just kidding about the tranny--I hope. Shifting is very smooth and it had a recent Volvo flush.)