93' xr650l-big bore, supertrapp exhaust, daves mods, removed snorkel, de-smogged, just over 18,000 miles. First off this problem only happens when on the highway in 4th or 5th gear, and only when the bike has been running for 10-15 minutes or more. When I get on the throttle the bike starts to pull, then begins to loose power and I can hear kind of a pinging noise from the engine area (sounds like putting nuts and bolts in a metal pan and sliding them back and forth). The bike does not make the noise if it is in neutral and I hit the gas, or when I am in the lower gears at high throttle. HELP!