This subject was covered in October last year in Spectator's Balcony in a thread titled "A380 Vibration". Various posters mentioned the same TO experience in 747, DC10/MD11 and A340 aircraft. The common feature is all these aircraft have one or two main landing gears mounted in the fuselage thus any vibration from a centre or body gear is transmitted straight in to the cabin floor and is fairly apparent to occupants around the main gear area. My own experience was it was nearly always out of balance tyre/wheel units often caused by uneven wear or flat spotting and was most apparent as the weight came off the wheels and the out of balance "rattled" the unloaded gear until the spindown brake system stopped the wheel rotation.