Most heat pump buyers consider noise levels when deciding on a heat pump. So lets look into this a little further.
The only noise produced by the indoor unit, is the sound of air flowing through the unit. If the heat pump is running at a high speed, as it needs to do initially to heat or cool the room effectively, there will so more air flowing through the unit, so naturally there will be more sound. As the room is cooled or heated to the desired temperature the fan will slow down and the noise will drop.
The indoor noise levels quoted by heat pump manufacturers are measured at the lowest fan speed of up to 5 fan speeds, or in the units "quiet mode". Running a heat pump on low speed or quiet mode will not effectively control the room temperature. A low fan speed mean a low level of heating or cooling.
When comparing different models of heat pumps, remember the air flow settings of the will vary slightly as set by the manufacturers. A lower noise level means a lower air flow, and lower level of heating or cooling.
All a heat pump manufacturer needs to do to claim the quietest heat pump, is design the indoor unit to run at a lower airflow rate on the lowest fan speed than its competitors and this will mean a quieter decibel rating on the brochure.
In reality running at the low fan speed or quiet mode is not an effective way to operate a heat pump.