Yesterday on my 2002 ex-l I noticed the same thing and found the ody at 45mph rumbling, I don't think this is sserious as I was not accelerating hard, just coasting. I think this noise is from the exhaust as the the engine is reving under 2000 rpm and is slowly picking up speed. If I am right the 5th gear kicked in and the engine is coping with the lower rpms and is making the vehicle rumble.