When the muffler has become sufficiently corroded it will no longer muffle the exhaust noise properly. The muffler brackets or rubber hangers can also break, allowing it to swing underneath the vehicle and hit the undercarriage. Some high end vehicles have vacuum or electronically actuated flaps that adjust the noise level, and when faulty can cause
rattling noises from the tailpipe, or excess noise when the flaps are stuck open. In the rare case the muffler is clogged, they engine may fail to start, stall, or run roughly due to excess back-pressure.