Right, my S4 has developed a super charger type whine. I was very concerned it was terminal but have today removed the drive belt and its gone away. This leads me to beleive its one of 3 things and I'm looking for advice from anyone who has had any of these fail.
I replaced the drive belt tensioner when I did the cam belt as the old one spun very freeley and felt like it had a bit of play. so its not this.
The Air con pump, this feels a little graunchy but this is with the clutch dissengaged. The noise does not change at all when the AC is switched on and off except that the revs change slightly so the noise changes pitch. The AC is functioning correctly.
The PAS pump, this feels fine and although I do get a slight singing noise when I dry steer its only when you turn extreamly hard without moving forwards/backwards. The whine does not change when the steering is used.
The viscouse fan or its bearing, again both feel good and I get no overheating. The fan has good resistance and the bearing feels smooth. Are these available as replacement parts?