rear end knocking noise on 2001 Ford F-350 Super Duty

when first stepping on the gas, I hear a knocking noise in the rear end. Don't hear it again, unless I start from stop position. Anyone know what that might be?

Asked by for the 2001 Ford F-350 Super Duty
check to see if your spare tire is tightened down. My spare did the same thing and dented my exhaust and I didn't even realize it until the cable broke and I lost the spare.
2 more answers
It could be your drive shaft or your rear end. I would get this resolved because something will break at some point.
to me it sounds like a u joint that has gone bad.