High pitch whining in shower

Recently started hearing a high pitch whining in shower. It's walk-in shower with single hot/cold knob. I removed shower head and still hear the noise (although not as loud). Not present with just cold but begins as I turn the knob to hotter water. I haven't tried adjusting the pressure, but the water pressure in our entire house is not very good. Neighbors report low pressure as well, so I don't think it could be a problem with pressure too high.

Any suggestions? Thanks.

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.

Yes, I read that thread. This is a walk-in shower with no tub spout.