A Plumbing Question Regarding Pressure Balance Valves

Just installed a Moen Posi-Temp pressure balance valve in my shower. While it effectively prevents loss of cold water pressure and resulting scalding death unpleasantness, it does so with a really annoying side effect. If the shower is running and somebody flushes the toilet or turns on the cold water, the valve emits a high-pitched whine, which goes away as soon as the other water source is turned off. Why? And how do I make it stop?

FYI, I’ve checked my solder points repeatedly; there are no leaks that I can detect, and the valve functions perfectly in every other respect.