[Lowfer] Can anyone identify this 28 kHz signal

[Dexter McIntyre]

While playing with the new VLF loop I came across signals in the 27 to
29 kHz area that I haven't heard before.  Sounds like data being
exchanged between several different transmitters on slightly different
frequencies.  I hear it best on the loop but can also hear it on a
random wire.  It may be coming form something locally here but if so I
have not been able to associate it with anything.

After hearing a series transmissions the frequency will be quite for a
while.  Over the past hour I've heard the signals about half the time.

I've uploaded a 1 min .wav at:

http://www.w4dex.com/temp/28khzunknown.wav

Recorded at 28 kHz with a BFO frequency of 2 kHz.

Dex