Install air chamber to cushion sudden-stop of flowing water.
Water flow coming to sudden stop is called water hammer, and will bang pipes, but also loosen plumbing joints, causing leaks.http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#banghttp://waterheatertimer.org/images/Flow-rates-water-hammer-600.jpg
Air chamber has to be vertical, and about 18" long, with cap on top.
By code each Hot and Cold line at each tap will have air chamber located behind the wall. So there are two air chambers at each tap.
In your case, you can probably add air chamber to your Hot and Cold lines and solve problem for both dwellings.
Water hammer is caused because water does not compress like air.
So when long column of water suddenly stops... the whole pipe-full of water comes to sudden halt... and that water could weigh 1-1/2 pounds ... and just like a hammer head moving, the weight of water bangs the pipes to dissipate energy, causing noise.
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