Rear end whine- could it be alternator?

Sorry if this is not specific enough for valid advice, but in the past 3 days my 2004 996 C4 has been making a whining noise from the rear. It seems engine speed related but is not so audible at idle. Peaks at around 1000-1500 RPM then gets higher pitched until road noise suppresses it. At its peak it sounds a bit like a circular saw! Tends to subside when the engine gets hot, but today it only quietened after about 18 miles. It's not the A/C pump- happens regardless of A/C being on/off, and it has been recently regassed. Happens whether in gear or coasting in neutral. Could this be from the alternator, or water pump?

Hi Neil I had a similar whine from mine about two years ago. I initially thought that it might be the gearbox and / or the alternator. I must say that I was really worried and did expect the worst. When I examined it with the engine lid open and the engine idling it was noticeable then. So I could discount the gearbox or any other sort of transmission problem. Using that long and not very reliable test of putting a screwdriver against various engine components and my ear, it appeared to be coming from the area of the alternator. I dismantled the auxiliary belt (polyrib) and before I did anything else I rotated one of the idle rollers and found this to be very slightly worn. Nothing too obvious, just a feeling of grating as I rotated it. Luckily I had bought one ( the right one too) when I got the new belt, picture at the bottom here. I replaced this and the result was no more whining! Two years later and all is well. By the way. If it turns out to be the alternator, take it to a good auto electrical repairer and the will change the bearings and bushes relatively cheaply (£100 ish) compared to the £800 plus exchange at an OPC. I hope that this may help you.

Thanks Peter- that's extremely helpful and does put my mind at rest somewhat! I think I'll pop to a local auto electrics company for advice ASAP!

Just had one of the rollers replaced on mine, exactly the same symptom as you described and I too thought water pump, alternator or worse, turned out to be the bearings on a roller and was replaced under warranty. Did the polyrib at the same time for good measure

Update on this problem. I took the belt off and checked the deflection rollers- some play in the lower one and both looking worn, so I changed both and the belt: now the noise is worse than ever, and is much more obviously coming from the water pump. Checking that, there is a lot of roughness when spinning it, so I guess I'll have to try and claim on the warranty, as it's likely to be a significant job, and beyond my DIY capability anyway! Thanks for all the advice; at least the belt removal/replacement was straight forward!

Of course, making sure there is fluid in the PS reservoir also helps ; Ahem! May need to investigate where it went as it was serviced 6000 miles ago. Still a little noisy but much much better now! At least I know the belt and rollers are good anyway!