mysterious noise in city during thunderstorm PITCHED DOWN.wav

So I recorded a random thunderstorm in Barcelona with my ZOOM H1, and when I analyzed the audio I noticed some activity in the higher frequencies during 20 minutes.
(you can see it in the ORIGINAL clip using the spectral view: http://www.freesound.org/people/bulbastre/sounds/132884)

I pitched them down 18 whole (the maximum pitching Audition CS5.5 does in a single operation) tones to make it audible - here's the result.

Any clue of where this sound could be coming from? Industrial machinery? Sewers?

I opened a thread on the forums for everyone to discuss:
http://www.freesound.org/forum/dare-the-community/17385/

***the research so far:

It was recorded in 48Hkz-24bit WAV. Audition automatically oversampled it to 32bits.
It was recorded with an autonomous ZOOMH1. No cables.
I've been using the H1 for weeks (I even recorded another long storm long ago) and I've never seen such a sound.

***This is where I live:
http://g.co/maps/e7qt2

No industrial thingies, no power thingies either, no secret government research facility that I'm aware of.

My window faces to the intern "garden" of the neighbourhood (see the map) - the direction where I recorded. There's usually children playing around, but it was a rainy day, so it was empty. RF anthennas... Maybe, since this parts of the neighbourhood are usually where the satellite antenas are. However, nowadays it's difficult to see this kind of antenas in Spain, since Digital Television and Cable Television have taken the market.

There is a major street -Numancia- on the other end of my house, but no traffic sound can be heard from where my room is. Also, I placed the recorder OUTSIDE my room, separated for a double-glass window that isolates sound and temperature very well. It is impossible for the recorder to pick up any sound coming from my room. In fact, I was probably listening to music or working with movie audio, and neither of both can be heard during the whole recording.

In Barcelona sometimes you feel and hear the underground passing by. Although there's a train and metro station near, I never in my life felt or heard them. Never. Besides, metro and trains deliver low frequencies, not high ones -as pointed out by AlienXXX, they DO deliver high frequencies, if in bursts-.
I need to recheck the original audio, to see if there's some low frequency activity accompanying the high ones, but I haven't noticed anything so far. I'm uploading the original audio, so everyone can research and experiment.

AlienXXX pointed out the fact that sound looses low frequencies when being transmitted through air at long distances. That could explain why there's a noticeable activity in high frequencies but low ones remain quiet.

And YES! There's more of this mysterious sound in other parts of the recording. I recorded 8 hours of audio, and I only checked 2. The sound appears twice, but I'm sure it's somewhere else in the 6 hours I've not checked yet.