I recentely had a knocking on my mk2 - same sort of thing, only knocked when i was cruising or slowing down, but not when accelerating. Was told it was engine mounts, CV joints, springs or bushes - so replaced them all one by one, and it didn't cure it. Also replaced the brakes, and it was still there. Only when the gearbox packed in did i get it sorted. Apparently it was a set of bearings, something like 'pinion bearings', which sounded like a big tie-wrap attached to your wheel, as it was affected by speed. This may help, and then again it may not, but seeing as it took me ages to sort it out, i just thought i'd share it with someone

ps
you don't want your gearbox to go..............really