Since picking up my Model 3 in April I've never felt the car was as solid as it should be.
I've taken it several times to the SC for help in addition to troubleshooting on my own. The air vent under my passenger seat was not attached. Secured it with Gorilla Tape. The rear shelf also seems a source of noise. I added padding under the fabric.
Fixed the belt fastener on the passenger side. The two plastic pieces that form the top part were not put together correctly. Felt taped anything that could rattle under the back seat. The car finally sounded and felt more solid on bad roads.
On hard hits I still had a rattle somewhere in the back I could not locate. Finally crawled under the car and discovered the composite shield on the driver side opposite the tire was loose. Taking photos of the shield from above showed a missing washer and nut for the mounting hardware.
Still two weeks needed to get a SC appointment.
See video below to hear the rattle.