Heat pumps are being used in more and more buildings for heating purposes. Particularly in the case of split air-to-water heat pumps, the outdoor unit is a source of noise pollution. The fan installed in the outdoor unit is one of the main sources of noise in heat pumps. The interaction between the heat exchanger and the axial fan results in an additional increase in noise emissions, which poses problems both for the certification of the units and for human health. Experimental investigations have shown which factors of the installed heat exchanger are particularly important for the resulting sound emissions of the axial fan. It was found that an inhomogeneous flow field leads to an increase in the tonality of the fans. The turbulence induced by the heat exchanger interacts with the leading edge of the axial fans, resulting in increased sound emission in the low frequency broadband range. Modifications to the heat exchangers that produce optimised flow fields for the axial fan have been derived from simulations.