posted on Oct, 25 2007 @ 05:37 PM
Judging by the sound on the recording, I'd say the wind is the unseen specter at work.
The outer two swings have a lot of movement, as well, but there is probably some combination of the way the wind passes through the structure of the
swing set and, perhaps, some sort of resonance in the upper support bar through it's vibration that accentuates the movement in the center swing.
[edit on 25-10-2007 by Tuning Spork]