Possible IBWO Double Knocks from Texas

I have recorded approximately 8 possible double knocks in Texas during 2006, some fit the expected sound better than others.

The following three possible knocks were purposely recorded hundreds of yards distant from the source sound.

Wave files

Around 7AM       The possible knock occurrs at 2 seconds into the 4 secound recording.
Around 8:30AM  The possible knock occurs around 2 seconds into the 4 second recording.  This was low-pass filtered to screen out                                   some songbirds.
Around 8:45AM  The possible knock occurs around 1.8 seconds into the 4 second recording.

The parabolic microphone was pointed across a river into about 600 acres of privately-owned tall bottomland woods surrounded by miles of  lesser height almost uninhabitated bottomland woods.  The knocks came from deep within those woods.
Abundant recent  IBWO-type scaling was documented from dozens of trees about 600 yards away from this listening site.  We have not visually checked the location where these three possible knocks came from for feeding sign.

These recordings were made on a Sony TCM5000 cassette recorder using a Sennheiser ME62 microphone pointed into a an 18 inch Saul Mineroff

Below are spectragraphs of these possible knocks, using Cornell's "Raven Lite" software.  These charts use roughly the same frequency and time scale as those used for the posted charts at Cornell's site.  Please compare these graphs and soundsto those at Cornell's site; http://www.birds.cornell.edu/ivory/multimedia/sounds/listening/doubleknock_list/document_view

Possible double knock at 7AM
Possible double Knock at 7 AM

Possible double knock from 830AM
Possible double knock at 8:30AM

Possible double knock at 8:45AM
Possible double knock from 8:45AM

One day, three of us were standing in the center of a feeding area with numerous recent scalings and we heard a non-typical double knock while the recorder was running.  The two knocks are almost twice as close together as they are expected to be.  Possibly this knock is not from a woodpecker.  I believe this is the knock given by an IBWO that has discovered us in the middle of his feeding area.  We have heard this rapid double knock before on entering the feeding area without much attempt on our part to be undetected.

Non typical possible "double knock" sound
Spectragraph of the non typical sound below

non-typical "double knock"
Possible alarm double knock