They're as noisy as an AC. So here in Arizona no one cares. But in Belgium this was an issue as they run more in winter and in winter sound travels further.
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.
Outdoor noise is less of an issue in the winter in big cities because windows are closed.