I have Toilet Flushometer hissing sound but is not the diaphragm. Even when I close the water for the toilet I still hear that loud hissing sound. What else can it be? Could it be the neighbors toilet on the apt above? The noise is somewhat in from the pipes in the wall. Or maybe is the flushometer pipe who connects to the main pipe?

Thank you

Does the hissing sound start when you flush or does it hiss all the time? If it doesn't come from your valve where is it the loudest? Can you furnish more details? Back to you, Tom

The sound stays all the time. Can't figure exactly from were the sound comes. It's like coming from the wall. It is frustrating and is very loud. It is an pre-war 6 floors building. I am at 4th floor. I don't know what other details I can give.

Well somebody better get on this ASAP because it sure sounds like you have a pressure leak inside a wall that's leaking 24/7. Somebody's going to flood out unless this is looked after. Let me know what is found. Good luck, Tom