What causes a high pitched whistling sound to come from the water pipes near the water heater and and pressure valve when a tap is turned on?

Air causes the whistling noise. I'm currently on a quest to eliminate the problem in my house (just had new water heater installed about a month ago, sounds like a choir of banshees when the water's running, toilets are flushed, washing machine or dishwasher are running,, etc.) When I figure out how to get the air out, I'll post a follow-up. In the meantime, at least you know what it is!