Rattling noise coming from the driver side of the car when I drove over 40 miles per hour.

It sounds like 1 of 2 problems but to start with jack up the drivers side rear of car and block front wheels with 2 blocks of wood or 2 bricks , put a stand under chassis and then your safe to work.
Release hand brake and spin rear wheels ..and then hold 1 hand bottom and top and try to shake the wheel.. any movement means worn bearings ..any noise when spinning means worn wheel bearings..If ok
get under the car and bang the exhaust all along and if the rattle comes when you get to the muffler you have you answer.
Let me know how you get on . RON