Okay, I ride my '90 XR600 HARD, on the street with street tires. I can't help it, I live minutes from one of the greatest concentrations of twistie roads in all of North America! So I'm fairly new to the XR thing and I treat it like my old liquid cooled CBR. That is, I ride it hard and don't worry about much else. Anyway, after a long hard day of riding (hot engine most of the day) the engine starts making a light tapping noise. I check my oil and sure enough it's 3/4 a quart low. Well stupid me, I ride to the nearest gas station to buy oil. That's probably not the stupid part, but what is... is I rode HARD to the nearest gas station. I filled it up with semi-synthetic which is what it had in it, then called it a day and rode the 75mph 15 mile trip back home. When I got home, it was tapping again. LOUDLY! I changed my oil tonight and started it up and it's still tapping, LOUD! Seems to be coming from either the upper cylinder area where the cooling fins are, or from the bottom front area of the engine just below the header tubes. TAP-TAP-TAP-TAP-TAP Does anyone have any ideas? Or is this a tear-it-apart and figure it out type of thing? How much would a complete rebuid cost me? Ballpark figure? Thanks!