Tick tock noise cam/followers

Can anyone shed some light on the noise i am getting from the cam shaft area on my domi 500, its a tick tock type sound when i turn the engine over by hand i have had this checked out by a specialist who said he had not heard this befor here sleaved the follower tubes changed the intermediate gear newcam chain and still the noise is there, inhis work shop he had a 650 domi he removed the rocker covers and turned this one over by hand and the same noise was there but quiter but this engine was all assembled with oil in the sump, he said he thought it is probably a charicteristic of these engines but just had not come across it befor can you shed some light on this