"whirring" in my experiences with the focus is 90% of the time due to rear wheel bearing failure. The bearing is pressed into the rear drum and you can buy brand new drums already setup with new bearings and dust collars for about $50/side.
My majorly huge hand-made Focus RS replica build:
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My RS conversion is For Sale: http://www.focusfanatics.com/forum/vehicles-sale/578185-sale-rs-mk1-conversion-2003-focus-svt.html