I've got a hammering sound with only one of the two faucets in the master bathroom and only when it's on cold and turned up somewhat. It doesn't have to be wide open to hammer, but it does have to be only cold. No problem at all with the other faucet, less than 3 feet away! What can I do?

Loose washer, either in the faucet or the stop valve.

Harold nailed it: that particular stem has loose washer. Remove stem, replace washer, reinstall - and you are all ready to go !

I agree with the above. However, If that don't vet it click on back band I have something else you might try. Regards, tom