Yeah, refrigerant levels are all good. The banging is coming the area circled in red in the picture attached. I'm wondering if it's just the water bypass check valve banging open and closing.The heat pump is a reverse AC unit. Have you had the pressures and refrigerant levels checked?
I would remove the side panels if you can and then can better listen and identify which internal component the knock is coming from. It should be more obvious if it is coming from the water pressure control valve or the compressor.
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