Unidentified noise
 

This is going to be a little vague I'm afraid as I'm struggling with it.

I have an intermittent noise which I struggle to describe. It's like a whirling sound, not air, not grinding. Happens when accelerating under pressure either up hill or if I put foot down unexpectedly. Must stress not every time. Very hit and miss!

Seems to be getting more frequent. Comes from within engine bay I believe. Has no adverse effect on the performance.

I'm at a loss

2010 (10) R-Design 185 auto 77k.

Thanks and sorry it's vague. Been wanting to ask for months but can't explain the sound