My GRiSO had a noise from the left head when cold that sounded like loose valve clearance , but disappeared late in the day when it was warmer. Tracked it down to a loose header pipe. Seems the factory "lock' washers do a poor job of it, and loosened again @100 miles or so. I used some stainless 5/16 split lock washers and replaced all 4 on both sides. Torqued again to 18 ft lbs. 25Nm. Seems to have taken care of it.