I'm getting a weird side-to-side vibration in a Citroen Champ C15 diesel, which comes in at about 40mph and goes away at about 50mph. I can't feel it through the steering wheel (although I've checked all the tyres for balance and lumps anyway). While it's happening, the gearstick is chattering from side to side and the whole van seems to shake side to side as well. Anyone encountered this, or have any ideas about what it might be?