| I don't think it is the rear brakes sticking (the noise doesn't sound brake-like) but I will suggest this to the dealer as a possible line of investigation. In terms of weather - it has been dry on all three occasions.
I forgot to mention that on two of the occasions I have been pulling out of a parking space, on a hill, that has required me to turn the steering wheel a fair bit (not quite a full lock but close). However, it is not remotely similar to the power steering whine of the r53 - totally different noise. The other occasion when it happened I was on the flat and reversed out of a parking space and then put it in first to drive off (I was still rolling back slightly when I pulled off) and heard the noise very faintly. | 
