Just installed heat pump systems (daikin) this december.

The exterior unit is basically silent even when there was a cold snap (below freezing but not northern alaska cold).

I suspect an interior air source heat pump hot water heater, being smaller, will be noisier, and likely less efficient.

I'd love to switch my 240v/30a water heater to use a 120v/20a service, but will wait a bit longer for the technology to mature. Ideally it'd have the heat collection part outside.

In the winter everyone has their triple-paned windows closed. Is noise that much of a problem?

Is triple pane common in Belgium? It’s not in the UK (as far as I recollect) and they’re colder than Belgium

I built a house in 2014 there. Triple pane. Pointless. You never recoup the cost.

Uhmm...I see a problem for countries where you have wall to wall connected urban environments...If all 60 connected houses on a street, install external heat pumps, it will add up.

Turns out cities are loud! In NYC, at least, heat pumps are far more quiet than the endless window unit A/C's, or larger traditional A/C's, in my experience.

Outdoor noise is less of an issue in the winter in big cities because windows are closed.

Most of the noise in cities is cars. One idiot on a Harley drowns out a warehouse of heat pumps

Sounds like an installation can be really tricky or you will end up with something like this: https://youtu.be/1rKNT7-42J0?t=1510