Yep mine has driven me mad with that noise. Roll over an uneven suface like a speed hump. Especially turning into another street with a gutter rise. Knocking noise, knock knock knock. It comes across to me like something knocking back and forwards or sideways.
i have done everything, taken out the spare, the cargo cover, spare cover, emptied the entire rear. Still does it. When I go fishing on a dirt road it is non stop going over anything. I have thought it was suspension related, had everything tightened, still does it. Today im going fishing and will find a rise on the road it does it then back onto it, get out and try rocking the body and listen if It does it.
Mines a 2 litre s im in Aus.
edit
Tried some things, still there. Could it be a fuel tank baffle?