My hot waster pipes make noise when the water is turned on. The noices is like a dull knocking sounds. I have a mixer tap.The pipe is hinden in the wall. Any help thanks

My hot waster pipes make noise when the water is turned on. The noices is like a dull knocking sounds. I have a mixer tap.The pipe is hinden in the wall. Any help thanks
A few more questions. Does the noise continue after the valve shuts off?
Is it a "bang" a "rattle" or a combination of both? Is your shower valve a one handled or a two knob valve? Regards, Tom