Just an opinion...

If the sound is really right at lift off, maybe it's the wheels trucks
dropping back. They are designed to hang back-end down. I could imagine that
they thump if the lift of is quick enough.

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Travis
"Kevin Chandler" wrote in message
...
I would suspect that it is the suspension unloading. The suspension would
have to be extremely strong to support the loads required. I would
suspect
that it is not the smothest (quietest) thing while in motion. It probably
thumps hard when it is loaded up (upon landing) and when it is
unloaded(take
off). You probably don't notice it on landing because of everything
stopping its downward motion when it hits the ground. It is probably lost
with all of the other noises associated with impacting a runway. The
suspension probably sticks a little and when it is unloaded and effected
by
vibrations, it falls to its full extended position on takeoff.

Just and educated guess.