i was riding to work yesterday and was getting this popping up in my front end... almost as if two spokes were popping back and forth over eachother and its pretty loud. i checked and none of my spokes were loose, my wheel was mounted correctly, etc, but i can't figure out what its coming from.

i mostly hear it going faster downhill while not forcing any pressure on the cranks, but don't hear it spinning the crankset or front wheel when its upside down or suspended.

any ideas?