In article ,
says...

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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I was thinking something like that too - except in some large airliners,
like the 777, they do not "drop" into that position, but they are pushed by
hydraulic actuators.

The sound I'm referring to really is immediately after lift-off. It is far too
early for the gear retraction - in fact, this make a distinct noise as well,
but several seconds later.

Thanks to all who replied - I'm still not sure I know exactly what this
noise is, but at least everyone is thinking along similar lines. . .

G Faris