It's more of a whirring/whining sound than a rumble.....I need to get the cover off like you guys said, and see what I can find. When I did the rear brakes, I had to take the axles out (it's a dually), and when I did, the old oil looked pretty foul (as in black). I have wanted to change the oil anyway, so guess it's time. Now what if I can't visibly see anything wrong? Should I just button it back up and refill, and see if that quiets it down? Or should I tear it down.....??
