The noise that is makes is barely audible from within the car. It is a very faint whistling / turbine sort of noise, and I have to turn it on and off to get my passengers to hear it, even with the windows open.
Additionally, there is a switch to turn it off if one hates it. However, after driving this car daily for a few months, I wish it were louder. I can't tell you how many times I've gotten stuck behind a pedestrian or biker who is proceeding directly down the middle of a street or parking lot, and has no idea I am behind them. I creep along at 5 miles an hour, and I can either honk at them (very rude), or just wait for them to notice me. They usually figure it out after 30 seconds or so, and many times they jump and do a double take when they realize I "snuck up" behind them. A little more artificial noise would eliminate this problem.