Mystery solved!  

Unfortunately the source was me.  The initial small
pulse is my front door opening, and the second larger
pulse about 3 seconds later was my front door
closing.  I was able to reproduce the phenomena
at will. 

Due to warmer weather, a window onto my deck had
recently been opened about an inch, and the pulsing
of the air from the house due to the front door
opening and closing was enough to cause the
spikes on my microbarograph.  The microbarograph
is about 10 feet away from the window, but changing
its position does not seem to make much difference
in the size of the spikes.  My deck is enclosed
on all but one side (dim ~20'x10'x10') so all the
air in the deck volume seems to be getting pulsed
by the window opening.  The simplest solution seems
to be closing the window, but longer term I need
to set up a spatial averaging array of soaker hoses
in a field.

Sorry for the false alarm.  No little green men this
time. :(

Dave  
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