Water running sound behind wall in bathroom?

You have a pipe leak ! If you are up to fixing it yourself first cut away the wall where the water is heard on the easiest side to get at. Even if you now call a plumber, doing this will save you a lot of money. If the leak is obvious (and many are not) then how you fix it depends on the material of pipe. If its PolyB then you must cut out the bad section and repair it with PEX and 2 Sharkbite fittings, if it's a copper pipe you can do this but it's trickier fitting everthing in the space.