How can I fix this problem.

I have a tub/shower combo and when you turn on the middle control knob for shower use water starts to drip behind the walls from the pipes. It leaks down the pipe to the laundry room below. I can see a little behind the wall from the room below.

Thanks for you help

JB

If the drip's coming from the stem just snug up the nuts on the stem. If someplace else let me know. I have put up a image to assist you. Good luck, Tom