It's not exactly loud, but it's noticeable enough to be annoying in a quiet room.
My monitor made no noise at all for the first 2½ months or so, and even when I put my ear to the back of the monitor with it on I couldn't hear the fan, then this just started happening out of the blue.
It's only happened 3 times so far, each time after the monitor was turned on or woken from sleep after several hours of being off/asleep. In every case, it's stopped after 10-60 minutes and does not come back while the monitor is in use, which is a bit weird.
For now I'm just going to keep using it and see how things go, but I'm glad I didn't sell my previous monitor yet, cos if this continues I'll either be returning it to Overclockers for a replacement/refund, or contacting LG to see if they can swap the fan out for me.