Strange fan-like noise in a Kangoo - any thoughts?

Hi,



I don't really know how to describe this fault so I'll simply write down as much info as I can in the hope it rings a bell with someone:



// 2006 Kangoo 1.5 DCi (car). Runs fine, starts fine, drives fine., no smoke, really lively etc etc.



In the past week or so a strange whirring noise can be heard - not at first starting and not all the time, but often after about 15 minutes driving and most noticeably when stationery / at traffic lights etc. Interestingly the pitch of the noise alters when the steering wheel is turned and it can get to the point of being quite loud.



On stopping the car and turning off the ignition the noise "spins down" like a fan getting slower and stops altogether after a few seconds. Come back to the car after 10 minutes, start it up and no noise is heard.



This happens on cold wet days like today or hot days. No dashboard warning lights, temperature gauge shows normal etc.



I'm stumped! Does anyone have any ideas? Or things I can try to diagnose the fault?



Thanks in advance,



Steve