My father has a sound coming from the water pump occurs only at idle and only when the bike is at full temp. It sounds like a bird chirp/whining noise. If you raise the rpm from idle just by 100 rpm or more it goes away. Rpm goes back to idle and noise again. Checked alternator and all is fine there. No coolant leaks either. Coolant was serviced 15k miles ago. Does he need a new water pump or can it be solved by doing an acid flush and adding good coolant?