Do you mean the entire car shudders or that there is something vibrating? If the car is shuddering, its probably only because the engine is rotating at the cars resonant (natural) frequency and the relatively stiff suspension isnt dampening it. My 1.2 did it, however my 1.9TDi did, now doesnt - this may be because mine is now bedded in a little better (58k Miles) and therefore turns over at a slightly different speed.
If it is a vibration, check for your strut brace fouling on the airbox, or the airbox fouling on something below (mine buzzes at 1100-1300 revs - very irritating but soon to be fixed)