Since you were in neutral and still heard the noise, I would guess that it is either u-joints(more known for vibrations when going bad), or a wheel bearing, there isn't much else other than the engine turning when in neutral. I would find a slope long enough you could role down it with the engie of and see if you still hear the noise, this would eliminate the engine. I would also get one of your friends(or you could) to walk along side and see if they can hear where the noise is comming from.

You could also lift the one tire at a time and spin it and see if you hear anything.