Catfish48

I get loud bang when I stop and put the truck into reverse and begin to back up. The more gas the louder the "Bang"! Does anyone know what the problem is and how to repair it. This has happened since the truck was new. I have had the truck to three dealers but no fix. All three dealers "reprogrammed" the transmission. One ford transmission mechanic said that it was normal. I have had many ford trucks and none have ever done this. What causes this noise and how can it be repaired. Any help would be most appreciated. 2010 F 150
Thanks in advance!