Vito 108cdi flat spot at low revs

[Pongo2000]

Hey guys. Another problem on the new wagon. I keep getting flat spots or moments when the power seems to hesitate when I am accelerating. It only lasts a second or two and the seems to be a raspy buzzing noise with it. Almost like a really quiet exhaust blow coming from the engine bay. There are no other issues and it drives fine. As soon as the noise stops the van is off again as normal. There is no smoke and the injectors are all fine. Any ideas anyone. I have just replaced the clutch switch as the old one was faulty but the issue was there before and the van revs properly now.