A little engine judder on the motorway

[A3car19tdi]

Hello.

I have this little a3 2009. 1.9tdie. Its had good service and now on 120k.

I've had it for 4 years and it's always done this. It has a little very regular shudder when travelling along the motorway at around 60 70 mph.  It's like every few seconds there's a little gentle shudder or vibration from the engine I can feel it through the seat and the steering wheel .  It's definitely not wheels or the likes because if I dip the clutch and coast along it stops immediately. 

It doesn't do it under load. Just at a steady constant speed and I can't notice it at high or low speeds.

Ring any bells to anyone?.  Every else appears fine at th moment.

[Steve Q]

Ha r you had a diagnostic check to see if any fault codes flag up? 

Could it be a faulty injector?