The rotation of the main bogies towards the front axle after touch down of the rear wheels, can cause a thump as the spoilers deploy, and the higher the pitch attitude at flare the worse it is. However the main second thump occurs as the centre gear touches down after the spoilers deploy, and the aircrafts weight is dumped onto the oleos. The centre gear lands after the mains as it is not stressed for landing. Landings tend to be smoother on the A330, which does not have the centre gear, (or a 340 with the centre gear retracted).
The 340-500/600 is better in this respect as the centre gear is landing stressed and lands at the same time as the main bogies.