rumble

('99 Lumina LTZ 3.8L) Lately I've noticed a rumble in the rear end (and I'm not talking about the one caused by too many bean burritos!:)). It happens as soon as I start the car, it rumbles for 3-4 seconds then stops and runs fine. I was wondering if this is the fuel pump or what?

If it's a low frequency rumble, it sounds like a muffler or resonator starting to go. Fuel pumps normally make a high pitched squeal/screech as they die. If the rumble is located in the drivers side rear fender area, it could also be the emissions vapor canister purging.

It is a low-pitched rumble. However I'm not sure which side it's coming from as it happens on startup which requires me to be in the car. I might have someone else start the car and listen to see if I can tell which side it's coming from. My car has dual exhausts, so shouldn't it have two mufflers? The pipe might fork off in the rear (one muffler), I haven't really inspected it too well to know.