also known as "airwaving", and inevitably referred to as "that sound you get from a jet/turbo prop plane taking off."
For DJ's, this is what happens when you run two copies of one record from the same starting point, at the same time, and at the same speed, the sounds cancel each other out, and you get a "flange" sound.
for the rest of you, a less comprehensive/comprehensible method of demonstrating the flange principle :
take your index finger (this assumes you have fingernails), tap it's nail on the table in front of you. (stop)
now take your middle finger, and tap it's nail on the table in front of you. (stop)
notice how both were simple tapping sounds, not very distinctive from each other (stop)
now, take both, and try and tap them as simultaneously as possible, eventually you will start to hear higher frequencies than you had heard previously, or it will sound different.