Get a stick, broom handle, long screw driver, start the motor, put one end on the valve cover the other too your ear. Probe the left cover and then the right cover, if it's in the top end you will know what side it's coming from. As Charlie indicates rocker or lifter most likely, the motor running smoothly just some clicking? You could wait until you get back home and adjust it. Be careful if you need help as to who you get that from. it is the Turkey Run and turkeys are lurking.. friendly FYI. ;-)