I'm helping a friend who has knocking pipes (Kitchen faucet only)Existed before, but worse since he installed a new Water Heater and kitchen faucet. I would have thought that any air would purge as the flow ran, but it still knocks...several thuds, usually with Hot water only.
Not sure if the pipes behind the sink are secured, but am trying to avoid cutting into the wall, especially if its no gaurentee.
It's no doubt pressure-related. You can sweat in an ordinary copper knock arrester - or two or three - at some point(s) in the hot - and/or cold - water lines if the line is copper tubing or buy copper ones that have a threaded brass fittings and do the same for pipe.
A very simple device that looks like a cigar and has a spring-loaded diaphragm or piston in it. When the water is turned on or off, the water line is temporarily "buffered" by the water's rush of pressure compressing the diaphragm against the spring and absorbing the initial shock or knock.
There are also whole house pressure regulators which are installed outside on the water main line as is enters the house.