As I suspected, from your comments the modern air units seem to have conquered the noise issue, back then it was a big problem!No you are correct, the underground source water to water heat pump, is more efficient. However,the additional engineering of groundworks and maybe sterilising a section of the lawn makes the Return on Investment decisions harder. Shrubs and trees will not grow well close to the grid of brine pipes, due to lowered soil temperature. Vertical bore 100 m holes are the best method ,but also the most expensive. Using a 200 watt air fan, over truck sized radiators is the most cost effective solution...which is why ASHPs are winning out.
Incidentally my air to water Heat pump makes less noise than my neighbours oil burner