2003 Chevy Impala ruble noise!

This problem was first noticed after transporting heavy furnitue in my car for a 2 hour drive. The noise comes up as a rumble noise when I step on the gas from a still position and I have noticed it as well if I am already moving at a low speed and hit the gas for a fast acceleration. I have already had it checked by a mechanic and he indicated that there is nothing wrong. I still hear the noise loud and clear and more during the colder mornings. Please advise! Thank you