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?

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.