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The pump is loud, or only hums

Too noisy. The manual's checks: there is water in the tank, and the circuit has been loaded.
Very loud, but water flows. The 2019 service manual: the pump has come out of its supports, so push it back in, or the intake tube is pinched or blocked. A retailer adds a missing mount screw.
A steady hum and no water anywhere, and priming does not help. The pump has failed.

A vibration pump is never quiet. One tester measured 69 dB on an EU E24.