Why is there a loud humming noise coming from the shower pipe even when it is not in use?

Why is there a loud humming noise coming from the shower pipe even when it is not in use?
What pipe are you referring to? What type of shower valve do you have? One handle or two knobs?

Thanks - the noise is coming from the wall where the shower head and valve are located. It is a single valve that you turn for hot or cold with a smaller switch underneath to choose shower or bath. We don't get the noise when the shower is in use.

We don't get the noise when the shower is in use.
That would put the vibration in the diverter. Open it up and check and clean the diverter and seat. The diverter's located in the front under the spout. TO access it the spout will have to be removed. Let me know. Tom