Low-pitch sound from engine?

This is for a friend, he's got a 99 Toyota Corolla Auto, the past 2 weeks, it's been making a low-sounding "grumble" upon acceleration. It only happens in D, in P or N, it doesn't happen. It happens at just around 1100rpm-1200rpm, nothing after that(we tested this by putting in D, and holding the brake and giving it gas up to that rpm). What could it be?

May I add that there is also some vibration at that rpm... don't know how else I could describe the sound, kind of a rattle too.. anyone??

Exhaust? Sometimes the mounts break and let the pipes rattle against the car somewhere.

Exhaust, yeah...also check the trans/motor mounts. If it's high mileage they could need some attention.

Thanks for all responses.. it turns out it is just a case of bad gas... it went away after filling up...