I just had that on my car! When there was no load on the drive belt it rattled, but as soon as there was load like turning the lights on or moving the steering wheel (So the power steering pump is under load) the noise went away.
On my car 2005 FR+ it turned out to be something called an altenator clutch pulley and is a fairly common fault according to the garage. While it was in I had the Auxillary drive belt and tensioner done at the same time. It was about £80 parts and an hours labour for the lot.