I'd like to increase my shower water flow in my top floor Master Bathroom. The house is a 3 level TH built in '93 with water heater located in the lowest level. The water pressure measured at the outside bib is around 40 psi. I have removed the water-saving devices from the shower fixture (brand new) and have checked the main pressure regulator to make sure it's fully opened up. The shower valve is an old Moen single knob that I believe might be the culprit. Would replacing the valve and trim with something newer help or it's not worth the effort? There is also a pretty noticeable noise heard right below the master bathroom anytime the shower is ran. The bathroom faucets seem to have adequate pressure and there is no noise when they are being used. Seems a bit odd. Thanks!

Your home should have 55 psi. many newer faucets are now ported smaller because the manufacturer has figured out that people are removing the water saving feature. No cure for smaller porting.