996 - high pitched whirring noise

[Dave Thornton]

My 996 C4S has developed a new characteristic - on start up there is a high pitched whirring noise which can be heard internally, coming from the left rear. It sometimes starts up when stationary in traffic and there is a slight dip in headlight brightness at the same time, indicating a load on the battery. It occurs with the aircon off, which incidentally is still ice cold - hopefully that rules that out. I wonder what it is?

[Ikebaddog]

Alternator bearing giving up the ghost would be my guess...