Vibrating Vitos??

[Lee2017]

So I have a Vito 2008 120cdi sport which has developed a body vibration at speed on motorway. Now this will last about 20 secs and happens rarely, maybe once in 100 miles.
So before I start stripping diffs and auto box I'm wondering if anyone has experienced the same.
Also I'm on cruise control when it happens thanks in advance