Gearbox clonk, Focus -05B european

Hi, when going from reverse to gear 1, or the opposite. I a have a clonking sound just when accelerating, ( not while shifting. ) Not every time but often, cant figure if it is the seat or the gearbox/clutch. it does not sound like anything is broken, just a short sound the way a non adjusted seat will do, except i cant feel movement in the seat. any ideas ? /Jorgen