I've noticed that when I start my '99 LTZ, I hear a buzzing sound from underneath the car. I only notice it right after the car is started. I can't really tell if it goes away after driving a while or if I'm just ignoring it. Anyway, it seems to be coming from behind the rear tires. Is it the fuel pump? That's the only thing I can think of. Any clue?? Is this normal on this particular year/model?