I'm building my first copter (stock Arducopter with 850Kv motors) and got the motors running just now. Soon I noticed that when they are running, there's a "ticking" noise coming from somewhere every now and then. I haven't been able to pinpoint exactly where it's coming from, but I suspect the motors themselves. I ran them individually, and all of them make this noise with and without propellers.
Here's a sample recorded with a cell phone: http://dl.dropbox.com/u/29136331/sound.mp3
Audio quality isn't great, but you should be able to tell what I'm referring to.
Is this a problem or am I just being too paranoid? Any ideas how to trouble(?)shoot further?