What kind of fan do you have? I have a HUGE, garage/industrial type fan that I use for littles who have trouble sleeping. I point it away from them so that they're not getting blasted with air, but they still get the benefit of a LOT of fan noise (and the fan also emits a loud but low-pitched hum that I think is really what does the trick). I've had parents call it my "magic fan" and run out to buy one themselves.
I do have to make sure that I'm checking on them very often, though, because it makes a monitor difficult to use.