[microsound] Zoom H4 tech query -- anybody else?

[John Hopkins]

Hey folks -- I've been using a zoom H4 for a couple years now, and have
been suspicious how a non-mechanical recording device -- in a really
quiet place (for example the Sonoran desert of western Arizona) -- and
the wave form of a short recording of a passel of birds in a mesquite
tree at dusk shows a cyclic 3-sec repeating pattern which seems to be
very low frequency something (sorry, don't have a spectral freq analyzer
on my machine).  I uploaded a screen shot of the waveform and a wav copy
of the file -- any feedback?  Has anyone else experienced this with the
H4?  I've seen it before on the files when recording in really quiet
places, and of course I suppose the best solution is probably a few
thousands spent in better field recording equipment, to be sure, but...
  what could be the cause?     In this instance I was not hand holding
it, it was on the ground in a stable place, and essentially no wind.
There is also a higher-pitched whine as well, faugh!

the sound file: http://neoscenes.net/09-03-092_birds.wav and a screen
shot of the file in Peak showing the approximately 3-second repeating
patterns http://neoscenes.net/09-03-092_birds.jpg

cheers,
John