My kids bathroom sink is making a hammering sound that seems to speed up in tempo the longer you leave the water running. The sound is coming from the wall behind the sink. I have 2 year old, all new copper plumbing in my 1 story (ranch style) home. At the time of the remodel, we installed a tankless water heater with a recirculating system. The tankless is located outside the kids bath on the exterior. When the kids sink is used, I also get a hammering sound sometimes ALL THE WAY across the house under the master bathroom (meaning that the sound is caused from the kids faucet). No other faucets in the house cause the hammering. I tried turning off the water main and opening up all the taps to reset the air gaps, but little water drains when i open any/all of the taps and when i close them all up and reopen the water main, the hammering problem returns to the kids bathroom faucet. Anybody have any advice? Thanks!