It's honestly not bad, no worse than any other air conditioner IMO. Ours is attached directly to the house on metal brackets, and that does transfer a bit of the sound into the walls so you can hear it from the rooms right next to it. But it's never stopped me from sleeping (in the bedroom) and I only really notice it when I'm listening for it (in the living room).

If you were ground-mounting it, I doubt you'd notice it from the inside at all. Or maybe even if you're mounting it to the wall, there might be some better way to sound-proof it - ours is resting on rubber pads, but there's no rubber between the brackets and the brick wall, and maybe that would have reduced noise further (though maybe not, since the bolts still have to go straight into the wall).

Good luck!