Whilst I have no 1st hand reports of vibration incidents (other than when I was PIC and the AC was a bit smaller than a jumbo and handled none too deftly in the beginning stages....
) ...
My cousin and his wife on return from the Philippines on an A340 also noticed what they described as a terrible vibration from 'under the plane' right after takeoff.
They were very concerned, spoke to an FA who replied,
'sometimes it does that - it's OK' ...............many miles later, at their flight's end, my cousin asked the captain what was that scary feeling ?' The Cap did not notice, but promised to 'have it checked out' - maybe they were blown off - or not.................
An EAL A&P told me long ago that the LDG Gear wheel/tire balance is often the cause of such worrisome but non-issue vibration reports.