I didn't want to take the time to go into a lot of detail, but I have solved "hissing" pipes in my own homes by throttling down in the inlet to the toilets. That was about 6 years ago, and so far no valve problems. The 10 psi or so drop across the valve isn't going to do it any harm. The hissing Alfred is talking about sounds like it could be high velocity in the pipes. So... either go to larger pipes, or a reducing valve, or go for the $0 cost option of trying the throttling. It won't cost anything to try.