My 04 SCrew 5.4 has done it since about 20k. Had rebuilt tranny under warranty at 56k- probably overkill but the dealer wanted to do it. Was fine til around 70k and started doing it again. Seems to be slowly getting worse. Not sure what to do about it either. I don't tow anything and I treat my SCrew with utmost care and maintenance. I just turned 100k last week.