Shower whistling

A couple years ago I converted a tub into a shower. I replaced the shower valve with a Moen. The way it works is you turn the knob to the left to turn it on then adjust left or right for temperature. I am told this is common for hotels. Anyway, a few weeks ago it started to whistle. It seems like adjusting the temp helps and well as the flow at the head. The tub spout seems to seal well. Does this sound like I need to replace the cartridge ( if that's what it's called) in the valve or has something else changed in the plumbing. Any ideas appreciated
Thanks,
J

 

If the tub spout is still there (sounds like it) and you are relying on the diverter in it to get the water to the showerhead, it's probably the spout.