When you next go to your car in the morning, , BEFORE starting it, lift the bonnet , then start it up, and youll probably find the fans are stationary, the "fan" noise, is most probably the sceondary air pump which warms the cat up (sounds like blowing, / vaccum cleaner) then it cuts out?
The fan housing is secured with reviets which come apart allowing air to escape , causing the noise you here, on here is a guide somewhere to taking it off and re rivetting it!
If it is actually your fans then this explanantion has been wasted, but hopefully not!
Dan