I did the valve adjustment on my CBR250R today as they were under spec after 3800 miles. I found something which I suppose could rattle although my bike never has before and I hope it doesn't start now as some others have reported after a valve adjustment. The pin that the rockers pivot on, and which must be removed to change the shims, is a slip fit into the head. You remove a closing cap over each one and they slide right out. When I put them back in the air that was trapped behind the blind end acted as a spring, pushing them back out at me as they move very freely. So I measured the depth from the end of the pin that was all the way pushed in, to the sealing surface of the head and found it to be 11.4mm. The closing plug is only 8.6mm long above the seal ring. This leaves 2.8 mm of play that the pin could use to vibrate one end or the other against the head or the plug and make a rattle. It would be a simple test to install some 3mm rubber between the plug and the pin to see if the rattle stops. If you had the right rubber that wouldn't deteriorate from the hot oil, it would be a safe, permanent repair.