We have a home with 1 shower/tub combo that has 3 valves (no pressure balancing or anti scald). When the tub is running you get full pressure and can balance the temperature just fine. When you turn on the shower though you have to turn the hot all the way on and barely have the cold water on. The shower then starts this loud fluttering noise until you turn the cold back up which gives you a cold shower or turn it back to the tub so no shower. Every other fixture gets hot water high pressure no issues. I replaced the hot cold and diverter cartridge's with OEM problem seems to be worse. Is the diverter the issue. this problem had gotten gradually worse since we bought the house 2 years ago, meaning hot water was starting to fade in shower but not this bad. I heard that it is potentially back flowing from cold to hot or that we need a spring check valve at the heater hot water pipe, air gap devices, old pipes (copper), failing water heater. arrgh. Please help get her off my back.🙄

 

Welcome to the forums.

Nothing jumps right out but if the water comes the thru the diverter and out into the tub just fine..... that leaves a clogged shower head. Some have fine holes that can clog and some have filter screens.

Try removing the shower head and just allow the water to come out the pipe.
Be careful it may spray all over the place.