Does someone have a clue what this sound could be?

This is not the actual bike, but is has the same clunk/knock when idling (and small reving) with the revs dropping a bit.

 

output shaft rivet issue ?

It sounds to me like intermittent weak spark in one cylinder. That's what I'd hope anyway, if it were my moto's engine doing this.  I'd check the coil terminals and clean them, check the HT leads and clean them, check the plug terminals and clean them then compare the spark plugs to each other. Diagnosing the condition of one engine by listening to the noise made by a different engine doesn't seem like an effective strategy though.

Try using a screwdriver and put it on different parts of the engine until you find the area that the sound is loudest. That will give you a starting point.

Hi for just a few pounds/dollars you can buy a Stethoscope on ebay, one of the handiest tools to have, noise finder/carb seter ect.And you can check your heart rate to make sure your still here

Thanks for the reply's, but this is a bike I'm thinking about buying, so I dont have it in my garage :/

The video under is of the actual bike. Some knocking and small drop in the revs. Could a bad breather hose and bad sync make this? (I know the hose had a small leakage).

How many thousand miles/kilometers on the moto?
Has the moto sat for some time neglected?
The owner have any maintenance history?

If you see the Z shaped crankcase breather hose is cracked, inspect the intake manifold rubber stubs, plenum/air accumulator rubber bushings and throttle body protection caps if original may all be suspect of leaking

Moto probably just needs a tune up and some new rubber

Sounds like leaky rubber to me, combined with the possibility of a partly-blocked injector. Nothing insurmountable, just a little time-consuming. See Volador's list.