Sometimes I hear a very annoying squeaking sound coming from the back, I discovered the problem it was the rear seats, they were not properly seated. If you play around with it the sound might go away. It happened to me several times, it sounded as if I was driving a 1980's van! My ML has 30/30/30 seats and can be lifted forwarded to allow access to the trunk area where I have third row seats.

Another time, a similar sound was coming from the front, the brake pads were the culprit!

Tabo