My 2012 CX-9 is making a loud rumbling / humming when accelerating. I also feel a vibration in the steering wheel as well as the seat. Took it to the dealer and they said it was the motor mounts; had those replaced. Did not fix the issue. Now dealer is saying it's the muffler. I don't hear any type of rattle like you would expect with a muffler. The car has 68,000 miles on it. Anyone else had this issue?