focus engine vibration?

focus 1.8 tdci 115 hp 111,000 miles.
engine idles smoothly, but it produces a lot of vibration only at 1200 rpm, beyond this it runs steady and smooth. fuel consumption ok 50-55 mpg, no smoke, plenty of power. replacing clutch soon as DMF in knocking and clunking, could this be the cause of the vibration. Its done this since ive had the car thought it was a engine characteristic, is it?
Thanks