blues riff_developing your musicianship

I was doing some random field recording of a thunderstorm with my ZOOM H1, and I saw some high mysterious frequencies that had some kind of pattern - rythm. I lowered them to make them audible. My audio editing program added noise by accident, so I ended up having a rythmic blend between a "natural" mysterious noise and digital noise.
I edited it a bit so it can be understood as a music track.
I plan to do more noise work in the future, if musically intended.

This is the original sound without the accidentally added digital noise:
http://soundcloud.com/bulbastre/mysterious-noise-in-city


Essay on noise number 1

So I recorded a random thunderstorm in Barcelona, and when I analyzed the audio I noticed some activity in the higher frequencies during 20 minutes. I pitched them down 18 tones to make them audible - here's the result.

Any clue of where this sound could be coming from? Industrial machinery? Sewers?