Our friend has an ‘02 Montero with a bad rumble in the front on the driver’s side. 182k miles on the drivetrain. Some shop told her it needed a prop shaft and maybe a front bearing. Personally, I’m afraid it’s the front differential. I have the car in the air on jackstands, and clearly, the sound only happens when the front half of the driveline is happening; no news there. But the way the sound is transmitted into the cab of the truck, it sounds like it’s coming from the diff. I drained a little bit of gear oil, which is probably original to the vehicle, into a clean bucket. The black crumbs are likely from crud around the drain plug. I have a Harbor Freight stethoscope, but I can’t confidence that way either. If it were my own vehicle, I’d start by draining the diff and filling it with Red Line or Amsoil synthetic gear oil.

What are your thoughts or recommendations?