Vibrating Pressure Balance Valve??

I have a single handle shower valve. When I turn the shower on, the PBV begins to vibrate loudly (about 4 hz) and the water pulses. It started about 1 year after I installed it. It only happens when I turn the hot water on. If I turn on another valve, it will stop.

I have drained and charged the lines and changed the cartridges twice. Finally, I just cut the whole valve out and re-piped in a new one (different make - Price Pfister). As soon as I turned the water on it started again. I then took the cartridge out and cleaned it, still vibrated. The pipes are secure and don't rattle. I can't see a noticable difference in pressure from hot and cold. The pipes are copper and I use city water. I have no other problem with any other valves.

Does anyone know what causes this or how to fix it?

it almost sounds like something is fluttering loose inside the copper tubing. A small ball of solder? bits of a loose washer? (I'm guessing)
I never came across this after replacing a second valve. If you had stuck with 1 valve, I would have suggested to clean out the pressure balancing spool, but now I have to second guess myself.

Thank you for your reply, I have also found (through trying different things) if I turn on the upstairs bath faucet, the pressure in the downstairs shower (the one acting up) increases, and the valve works perfectly fine. The 2 bathrooms are directly above and below each other. It is not a big house (back split). Could the upstairs shower valve cause the downstairs shower valve to vibrate like that? It seems strange to me. Any help is appreciated

jjohn73 said:

if I turn on the upstairs bath faucet, the pressure in the downstairs shower (the one acting up) increases,

Click to expand...

That is strange, if laws of physics hasn't changed since my high school days, I would have thought that the pressure would go down.

The only thing that could decrease the pressure, is something that is abstructing the flow. I can't quite comprehend why the pressure increases when the upstairs bath faucet is on.

I agree, it is the most bizarre thing I have ever seen. I would think it was air in the pipes, but I have drained and recharged so many times and I have even opened all the valves in the house and let them run. I am almost ready to just sell the house.