I recorded mostly nothing, then normalized it, so it was mostly loud static. I used audacity's noise remover to make the sound and it's wahwah effect to make it move across stereo space. Very interesting, good for static and alien voices.
My remake of the iconic hoover sound commonly heard in hard house. A lot of reverb in this one. Made using the sawer soft-synth in fl studio 10. The key is c5.