Sirs

Forgive the title however, let me try and explain the issue.

I'm getting what sounds like a loud fan from the engine. My wife describes it as sounding like a bus when it sets of! It's intermittent in so much as it comes and goes over a period of several miles and is more noticeable at slower speeds due to the lack of wind noise etc.

Arrive at one set of lights and she's fine, then pull away from the other and whoosh, it like a big fan going off under the hood.

The car seems to be driving fine with no loss of power and she's a 2011 Vogue with 50,000 on.

Any help, much appreciated.

Could be viscous fan