Odd Noise

Driving very slowly through the joyous South London traffic over the weekend this happened;

1st gear, idle-speed revs, car moving slowly but surely then a sound that I can only describe as the alternator "on a cold day" appears. Didn't happen with the clutch down/transmission disengaged and only in 1st gear. No odd noises at all on the blast down the M23 home either.

Didn't do it this morning so I was pleased but does anyone have a thought as to what this might be? Sounded a bit worrying but as the car is nearing it's 30k service I'll probably get the dealer to look at it.

Unless the transmission locks-up that is...

Thanks