Updated: Oct 1, 2021
In the old days working for Calorex we made some noisy units, the classic 10 range was a great performer but a noisy beast, I guess the legacy has stuck?
The latest generation of heat pumps including the imported Calorex units are fairly quiet, there will always be a certain amount of noise generated so careful consideration of siting will always be necessary with this type of equipment to avoid complaints from neighbours.
Certain manufactures perform better in terms of noise levels than others, this is particularly evident on larger units that have two fans running slower than rivals with one larger fan running faster.
One of the quietest heat pumps I've heard is the ultra quiet 'Norsup PX' range it even has a silent mode...