Air-to-air pumps will probably always make some noise because air has relatively small thermal capacity, requiring relatively big air flow in order to transfer

Air-to-air pumps will probably always make some noise because air has relatively small thermal capacity, requiring relatively big air flow in order to transfer any meaningful amount of energy. There also exists heat pumps which transfer heat to water-based central heating system (radiators / floor heating), eliminating the noise issue. The heat source can be either outside air or liquid circulating in soil/well (possibly supported by solar). All these combinations are relatively common in Scandinavia, air-to-air pump being most popular because of quick ROI (small initial investment) and easy retro-fitting, althought practically all new houses are built with water-based central heating (the heat source being either of electricity/solar/wood/air pump or any combination).