Water leak
Does the pump cycle on by itself when no faucet is open? And not connected to city water. It shouldn’t unless there is a leak on that side of the plumbing.
It’s not as easy as it sounds but connecting to city water and opening the faucet to pressurize the hose and then close the faucet and see if it holds pressure. Can’t have any leaks at the connections or faucet or an anti back-flow device. Not a definite diagnosis procedure but it might work to find a leak.
Most of the plumbing is accessible from inside by pulling out drawers or removing access panels and the little false walls under the sinks.
Being a few years old, always replace any cone washers if you take any connections apart. (I recommend)
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