I've noticed this sound for some time now (it may have always been there for all I know and I'm just now noticing). When I first start the car, there appears to be a tapping sound emanating from the exhaust pipe - just ahead of the rear wheels (near center of chassis). It's very periodic (once a second or so). And only lasts a few moments then stops. Initial thoughts tell me its emissions related (yes, thinking it's deliberate) because the sound either is there, or it isn't. It doesn't fade and it doesn't vary its timing.

Anybody know what this could be?

Thanks.