xtz 660 engine rattle

[stevie_g]

Hi All,I have just joined the forum hoping someone can give me some helpfull advice regarding my bike.I went out for a ride to Wales on my xtz 660 today and while on a dual carrigeway travelling at about 80 m.p.h the bike suddenly lost power and then picked up again,i pulled over and checked it but found nothing obvious,i carried on and noticed that the engine is making a rattling noise,not quite a tappet noise,more like something loose inside the head,power is fine and the noise does not get worse at higher speeds or under acceleration.I am beginning to think i am jinxed with xt's as this is exactly the same thing that happened to old xt600 about a year ago,i pulled the head down but found nothing and sold it as spares or repair.Has anyone had this happen to them and know what the cause is?The guy who bought my old xt has not looked at it yet but suggests that it could be the balance shaft bearing,is this a common failure?
thanks in advance Steve.