When I pull off slowly I get a knock from what feels like the front wheels (which I believe relates to the wheel revolution) upto about 10mph then it's fine after that. It feels like I am driving over a series of catseyes. If I pull off quickly it doesn't do it - it is mechanical - so not the tyres. The car is an 05 1.8T Cabrio Quattro - manual.

Any idea what could be causing it?