Assuming that it is a pressure balance valve, (and I don't know for sureif it is, but it probably is) then there is a spool valve that moves back and forth as the incoming pressure changes to keep the balance of hot to cold water you've selected the same. Its primary function is to compensate if someone flushes a toilet, or something changes the overall pressure on one supply or the other and keep from scalding you. You may be hearing that moving. Are you on a pump and well or city water? I'm guessing here, but if on a well, you may be getting fluctuations while the pump is running, and those minor pressure variations are causing it to move thatvalve. Now, this could be totally bogus. You could have a little piece of debris in the valve itself rattling around as the water is flowing.
I'm sure someone else will have some more enlightened ideas...interested myself in what is the real answer.