Brand New 2001 400EX (1 month old - past break in period)makes a singular clicking noise every time you rev it up after engine reaches operating temperature. Talked to several service reps at dealer, head of service, GM of store and Honda Corp - none of them have heard of this problem before. It does not make this noise all the time only when the engine is hot and you have been through all the gears. Dealer serviced 3 times already!!! Did the following work - valves have been adjusted, dealer replaced spark plug,cam & compensator, starter clutch, piston pin, 2 different gears, clutch outer casing, 3 gaskets and a bunch of bearings. Run 91 octane gas only (highest I can buy here in Colorado other than racing gas)... Almost sounds like someone is tapping the engine case with a screwdriver. Can't pinpoint exactly where its coming from but you can hear the noise over the engine.