Toilets, Sinks, Showers, Tubs and Disposals - Shower Control Valve?

For some reason I am now getting a high pitch squealing noise when I turn my shower on. It is a simple handle that rotates counter clockwise from cold to hot. It only squeals when I get over to the hot. Now, it seems to also heavily reduce water pressure as I move into the hot. It never lost pressure but now when I go to have a hot shower, it comes strong on cold and as I turn to hot it loses pressure and makes a squealing noise.

No other sinks or faucets lose pressure or squeal. Im thinking its the control valve in the wall. Any ideas, pointers....

Also, I dont have replacement tile for this as its a small hole behind shower plate. On the backside of my shower is the bedroom closet. Would it be best to go through drywall in closet and then patch it up from there afterwards?

Thanks,
KDC23

You should be able to replace the cartridge in the valve body withought removing tile. Unless the tile person put it too close. Take the handle and trim plate off and see.