Intermittent humming/whirring sound from rear of S60

[MOC12]

Hi,

There is an intermittent humming/whirring sound coming from the rear of my 2012 S60. I think it might be coming from somewhere around the right, rear wheel. The sound occurs intermittently:

when the engine is running and when the car is moving or stationary e.g. waiting at traffic lights with the engine running.

The sound typically lasts for between about 3 - 10 seconds and it occurs randomly - sometimes it might occur every 30-60 seconds; sometimes it might only happen once every 5 minutes.

I think it must be an electrical component as it occurs even when the car is stationary, possibly something like a fuel pump (I don't know where that is located on the S60) or even the electric hand brake.

Does anyone have any ideas what it could be, or how I might be able to track it down?