Ford Focus 1.8TDCI vibration at 1800rpm



I have a 2006 Ford Focus 1.8TDCI which has a vibration when the engine reaches 1800rpm, but then stops after the engine passes 2000rpm. I can hear the vibration and feel it through the steering wheel.

I have also noticed that it comes back at around 2600 - 3000 rpm.

It does not seem to be related to road speed at all - only to the engine speed.

I have not had the car for long and any help you could give would be appreciated.....if I take it back to the dealers I would like to have an idea as to the problem (less likely to be fobbed off)

Thankyou in advance.