Sounds to me like main wheel vibration, after wheels leave the ground they will shake the landing gear.
When gear selected to up, the main wheel brakes are applied to stop wheels spinning, this is very nice.
With 18 wheels, they will at times balance each others vibs and the totel effect will be less, but at times they will act together due to the random effect, when they act together, vibs will be high.
I don't like those vibs when I travel, so I book seats on the upper deck or between doors 1 and 2.
Have a nice flight......