I have a very loud, high pitched whistle in my upstairs shower that seems to have gotten worse and worse lately. It's loud enough at close range to make me worry about ear damage, and it certainly wakes up others in the house -- both big problems.
Other potential clues: The whistling sound is somewhat worse at partial pressure than at full pressure. I recently replaced the shower head, but I don't think that was the cause. My current theory is that is has something to do with the shower valve, but I don't really know what would be causing it and am not sure how to fix it.
Is this a shower in a tub? or a dedicated shower? If in a tub with a diverter in the tub filler spout, that is probably where the sound is coming from. Most spouts do not have replacement parts available for them - it is usually fairly easy to replace the whole thing, though.
If it is a dedicated shower valve I've heard that a loose washer can cause this. If so, then it can (usually) be repaired.
One of the pros may have some other ideas.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013