knocking noise frm gearbox

[Whitegv]

well i decided to keep the car, but i think i like it more than it likes me.

any ideas what the following could be??

just come back from mrs house, pulled off and there was quite a loud knocking noise, however this noise is only there when the clutch is out.

if i accelerate, it does it, but as soon as i let off the accelerator it stops

if i put the clutch in whilst moving, the knocking stops, and if i put it in neutral whilst rolling it stops.

anyone any ideas?

it had a new clutch about 2 months ago if that, and i changed the gearbox at the same time as a synchro had gone in mine, the gears are sometimes really tight, especially 1st and 2nd. could this have anything to do with the knocking?

its been fine up till today.

cheers