Rattling in new copper pipe- hot only


I had a bathroom added to my house as part of an addition last summer. The copper hot water pipe rattles inside the wall; at least it does when the valve under the sink is not fully open. My wife likes to turn it down halfway or so. When the valve is half closed, running the hot water in the sink cause the pipe to emit a loud "chatter" in the wall. The contractor who did the job, and his plumbing subcontractor, have apparently told my wife that the valve should be open all the way for normal operation. They say of course the pipe will rattle when the hot water valve is turned down. I suspect the pipe somewhere inside the wall was not properly secured. What is the real story here?
Greg
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There is no practical reason not to open the valve fully. The hot water passing over the partially opened valve is causing harmonics and vibrating the pipe. Hot water tends to do this while cold water hardly ever does. Sometimes a defective or worn washer in a sink faucet does that also. A new valve may or may not solve the problem, but either opening the valve or replacing your wife definitely will.
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