A couple of 340 pilots on this thread mention about the big thud coming from contact of the centre gear. My outfit operated a 340-300 for a while with the centre gear inhibited and yet the same usual thud still occurs. The reason is believed to be that it's easy to grease the back wheels of the bogeys on (like the 330) but the main thud is actually from landing the front wheels. When you realise that you can then make out the 4 different stages of an A340 landing, ie tickle (back bogey wheels), big thud (front bogey wheels), thud (centre gear), thud (nose wheels)!