T5.2 burr sound?

[brasstinman]

Hi, we noticed a burr sound coming from the steering only a few days ago. We will be driving down a straight smooth road and then you hear this burr sound, almost like a power steering sound when you over turn the wheel which we don't do BTW.
We have the 2011 T5.2 132kW 4motion with front diff lock optioned in. It's got about 17k on the clock and has had it's 15k service.

At first we thought it might be the traction control but it happens when the road is smooth and there is no obvious lack of traction on the road. Well maybe the Multivan thinks there isn't and is compensating somehow.
Can't feeler hear anything wrong in the steering when we move it.

I looked up the book and can't see anything about power steering issues. Can't even see the power steering fluid in the engine bay or in the book.

Has anyone heard a similar sound from their T5.2?