We are still hearing a light rattle when the shower or sink faucets upstairs are shut off quickly.
To summarize, we have done the following:
1) replaced the regulator and set the pressure at 50
2) installed expansion tank
3) replaced sink faucets
4) replaced shower cartridge
5) visually checked the pipes in attic which connect the upstairs bathroom to the water heater - the rattle is right before the water heater
Really can't think of what else to do. With a light rattle, is it likely to cause damage to our plumbing? I'm thinking we may have to learn to live with it. It's not my preference, but I'm at a loss. Thank you all for all your help.