You don't describe the noise so this may be may be a dumb question, but are you sure the pump is off when you're connected to city water?
That being said, we get kind of a light whining noise when connected to city water and we turn on a faucet. It is our inlet check valve. The check valve keeps water from flowing out the city water inlet when you're not connected to city water. The opening through the valve is rather small, so higher water flow can sometimes cause a noise. Partially closing the water valve at the pedestal as trackman suggests may help.
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