First off, this is only what happened with mine so its probably worst case so dont worry until properly diagnosed, but mine was doing this for some time, same situ in that the clutch and release bearing were new.

It turned out to be the layshaft in the gear box.

Once it had failed

(on the motorway) I was told

(although I never opened up the box) the shaft had completely collapsed, even out of gear with clutch to the floor it still judderd the car forwards.

.

but anyway, it started as a knocking.

Ended up fitting a recon box.
