Cockpit noise

Hi all my merc e240 has been making a rattling sound in the cockpit upon ignition off . Recently I hit a pothole now the sound became worst when driving. I went to the mechanic to have a look at it he could not find any fault form the engine side . When I look at the exhaust pipe underneath it shows lla small crack on it however the crack doesn't look that new.
The noise can only be heard from the inside of the car upon ignition on and when driving