My 1999 400ex makes a humming noise that seems to be coming from the clutch while it is warming up. After the beast is warmed up the noise stops and remains stopped until the next cold start. I don't notice any clutch performance degradation, but the noise is disconcerting. Any ideas or suggestions?

 

I think it might be the oil pump. Is it kind of a "whine"? I have a 1999 and I don't have this problem but I saw a 2000 model last weekend that when cold had a "whine" as you rev the motor. Went away when cold... Hope this helps.