Hyundai cold reverse rattle
Some 2011-2012 Hyundai Elantra vehicles (equipped with automatic transmission) may experience a rattle noise from the front of the vehicle with the brake applied, in reverse gear, after a cold start. The noise does not occur after engine warm-up. Do not replace the transmission for this condition. Follow the repair procedure and replace the front muffler.
With the vehicle raised on a hoist, and with the engine running and parking brake applied, shift the transmission into reverse gear. Check if the noise comes from the front exhaust muffler. Use a screwdriver to press against the exhaust hanger and again on the flex pipe. If the noise stops, replace the front muffler.