Rear-Passenger making LOUD squeaking noise..sorry new to this forum

Currently I own a 2001 BMW x5 3.0i. My rear passenger side is making some sort of a hardcore squeak. I can hear it coming from DIRECTLY under the backseat. I have no idea what it is...I just got front and rear brakes done so that's not it. It sounds like a metal to metal squeak. Usually it happens when I go over a road that is not perfectly straight as in some bumps. More likely when its cold. Please