I wouldn't say it's very loud, but it does make the typically expected A/C compressor and fan sounds. It's not annoyingly loud, unless it's sat up over the winter and hasn't been run for months, but even then it tends to settle out more quietly after 30 minutes to an hour of running.

Ours is a Hayer HPQ10XCR-P3, though I'm sure there are a many models and manufacturers out there of similar devices. I can't speak for the sound qualities of all of them, but I would recommend running them at least for an hour every few months to sling the lubricating oils around.