Rear rattles

Hey everybody!

I own a bmw f31 330d msport and something has been bugging me for the past 2 months: the rear rattles when I go on top of bumps and uneven surfaces. I took it to the mechanic and we tested together, when I would press on the brake pedal the mechanic would kick the rear tyres and you wouldn’t hear the rattle, but when I let go of the brake he would kick it and u would hear it again. So we changed the brake pads, the guy told me the brake pads were too short(??) (not used up) and it stopped for a few days and then it came back. Is it the suspension? Had anybody dealth with this before? How do I fix it?

Thank you for the help :)