Vibrating/humming pipes

I've read numerous posts similar to this, but couldn't find an answer I need. When the washing machine, shower, dishwasher, etc. are turned off or stop using hot water, the pipes make a loud humming, vibrating sound. They don't bang around, just a loud groaning noise. I've seen numerous references to "checking the pressure regulator" but that doesn't tell me squat. Check it for what?!? Hope someone can help, as it started a few weeks ago intermittently, but has now gotten progressively worse.