Every so often I get a consistent clunking/thudding noise that increases with the revs.

To give you an idea, the clunks happens akin to every revoution of the wheel at around 20mph then gets faster until it blurs into one. Then I'll slow down to check again and its gone??

If I dip the clutch the noise goes (whenever I can get it to do it).

Any ideas? with it being consistent its got to be drive chain related?, I originally thought the propshaft might have been catching something, but even doing 5mph that would be a clunk, clunk, clunk rather than a clunk..............clunk..........clunk if that makes sense?

I've jacked it up but can see a thing? Any ideas?

Thanks