Someone a few days ago asked almost exactly the same question...his problem was the diverter on the tub spout and is a fairly common situation. Play with that a little, and if that changes the sound, you've found the problem. They're usually not repairable, so replace it. One of the long-time pros here finds the Delta spout with the pull-down diverter works quite well and lasts a long time. To activate it, you pull down on a ring around the outlet of the spout, when you shut the water off, it springs back and opens up the tub spout again.