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.