One cause of rattling noises coming from the underside of a vehicle is a component of the exhaust system being loose. This could be a poorly secured heat shield, a loose muffler, broken hangers or ill fitting pipe gaskets.
Loose U-joints along the drive shaft, failed bearings and worn CV joints can also produce similar rattling sounds. Further rattling can also come from the wheels when brake calipers are loose. If the rattle is heard after hitting bumps in the road, it could be caused by a bad ball joint or another problem with the suspension system. Unusual car noises can serve as initial warning signs of automotive problems. A proper diagnosis should be sought from a mechanic to determine the true cause.