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