Q: When I'm sitting in park my truck makes a loud bang sound

Depending on the sound and the location of the sound, this may be related to a misfire/backfiring problem. Engine backfire is caused by an imbalance in the air/fuel ratio being supplied to the motor and will occur in one of two places. A backfire in the intake manifold is caused by a ratio that is too lean or not enough fuel. A backfire out of the exhaust system is caused by a ratio that is too rich or too much fuel. As there are many possibilities here, this may require the skill of a qualified mechanic such as one from YourMechanic to come to your location and diagnose your backfiring problem.