The pump has a vertical pipe connecting it to the main outtake pipe leading to the sewer. There is a brass check valve in the connector. When it starts up, I hear hmmmm, then HMMMMMMM, and then a ...

A diagram of a heat pump cycle with two capillary metering devices with their check valves. (Figures are from Refrigeration & Air Conditioning Technology , 5th Edition, by William Whitman, William ...