Shower pressure dropped and noise increased

Hello,

Just a few days ago something strange happened, wondering if you can help diagnose.

The shower pressure dropped quite a bit, and our shower is now noisier than it used to be (kind of like a high pitched whistle).

It went away briefly yesterday when I switched it form tub to shower mode and back, but today the pressure is low again.

This seems to be a quick change (not gradually building over time), so I'm wondering if it's in the gizmo that switches the water from the tub to the shower? I have more recently been switching from shower to tub to fill buckets for an evaporative cooler. I've included a picture if it helps you guess what brand, etc I might be dealing with. FYI there is, maybe not surprisingly, an H on the hot, C on cold, and a triangle on the middle knob.

Is fixing that something easy for a very low skilled DIY person, or do I call a pro (or is that too small of a job for a pro)?

Thank you!