Re the chap whose Fiesta had a whistling fan belt; It's not the slipping of the belt that matters so much as the fact that it's become "glazed" and it's whistling like when you run your wet finger round the rim of a glass.
On mine I isolated the cause to be the varying drag caused by the alternator when I switched the rear heated screen and headlights on and off, the whistling went on and off too. A single drop of WD40 on the belt and the noise stopped for good. The alternator still turns but if it's inertia causes it to spin on after I decelerate or lag behind when I accelerate then it can skid just as before, but with less wear and NO NOISE. I carry a couple of spare fanbelts and an adjustable socket spanner anyway.
One of these days I'll get around to fitting a light in the engine bay.
Tom.