e90 squeaking in reverse

Hey Team,

Ever since i cam back from December holidays i have been getting an annoying squeaking noise coming from somewhere at the rear when i am reversing the car.

The noise immediately disappears when i put the car in neutral or when i drive it. I took it to my mechanic and he did a physical check and they said its not the brakes. He said it could be the suspension...

This is a big word to me...

Any thoughts on what this could be?

the car in question is a 320i with 135k on the clock