My X5 does the same thing and but I believe it is the clutch fan on the front of the engine. Not sure if it needs replaced becuase I am not overheating but it seems to me if the fan is only loud during initial start-up and then everyone once in a while that it is the fan.

Here is a link on fan clutchs.

http://www.popularmechanics.com/auto...t/1772922.html

I do hate that whirring noise, the entire neighborhood can here it