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What is the problem? are the drains backing up or is the water not going down?
The most common sources of this type of noise is from an outside faucet that has a hose attached. Do you have a hose attached to an outside faucet? if so try removing it and see if the noise goes away.
There's nothing on a tankless heater that's likely to cause this or any type of noise when you turn off a faucet. Other possible sources are a PRV (pressure reducing valve) or a back flow preventer on the water main. Since the water was turned off when the heater was installed it's possible some sediment that was stirred up when the water was turned back on could be fouling one of these devices.
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