Why does my Galaxy S3 make a noise when I put it down?
Well, I've figured it out. Sort of. I habitually put my phone down on top of my wallet, which contains my NFC-chipped credit card. It turns out the phone actually makes that noise when it finds an NFC chip. If I push my credit card against the back of the phone the same thing happens.

I'm still not sure what it really means, though.