X3 3.0d rattle/whirring noise on cold start up

Help
I have a3.0d 97 x3 and when cold near freezing or below a noise sounding like a whirring/rubbing noise can be heard coming in the general area of the alternator. After a few minutes and for the rest of the day the noise disappears! Has anyone any ideas as to what it maybe. Curious that once the engine is warm it cannot be reproduced.

Can you provide with more information?
Does it sound like a bearing noise, loose belt, pulley?

I had a similar issue with harmonic balancer. The inner rubber was cracked which caused the belt to stop every time I would turn the aircon on.
Have you tried using a different diesel provider?

It sounds like a noisey bearing very rattley for about 3-4 minutes then just dissapears

Belt tensioner?