Transmission adjustment question

The reverse lamp switch is mounted on the shifter either in/under the floor console or low on the steering column. If the trans fluid is low and/or the torque converter leaks down while parked you'll get a sudden reverse engage/bang. Try this, go to neutral first and allow the front pump to fill the torque converter ( a minute at most ) then go to reverse and see if the bang is there. If it is the drive line is suspect.