At my mom’s place — air-source heat pump, double-paned windows — I can’t hear the thing at all from inside the house, and can only hear it if I go on the side of the house where it’s operating, which doesn’t get a lot of foot traffic. You can hear the fan there.
Generally, I haven’t heard people complaining about it in the US. I have seen some people talk about it recently in the UK, which is in the middle of a push to transition to them, and I’m wondering if that’s because townhouses are more-common there, with houses packed closely together.
I understand that you can get noise-reducing enclosures:
There are also water-source heat pumps. I don’t know how the noise differs, but I’d bet that it’s quieter, because you’re moving water through a pipe rather than a lot of air. However, their installation cost is considerably higher (though their energy efficiency is also higher).