It’s the air displacement that causes it, happens to me while playing. We finally put a couple guys all over the net and served a couple and the were watching from about a foot or two away, the balls wasn’t near the net, a couple inches about, but they all heard the ‘brushing’ sound.
I know though that Andy Roddick has actually set the sensor on the net off and it clearly didn’t touch it. The official told him, if the sensor goes off it had to hit, the wind wouldn’t set it off. If we’re going to have the challenge system, I say allow a challenge on a call like that. =/
Anyways, the speeds of the serves do play a roll in the ‘brushing’ sound and more air is being displaced than people tend to think. =)