I didn't hear it at all, but it was around a campfire and people were laughing and talking.
I imagine there must be some fan noise, and if you were solo, or in a hunting blind, it might be a slight annoyance, but when the skeeters are bad, I could put up with a little low level hum ;)
I went too buy one after that encounter, the local ACE hardware said they couldn't keep them in stock.