Strange clattering under rear wheel arch?

[Walton]

Driving along today my R400D made a strange clattering sound - like a load of bolts had been kicked up under the left rear wheel arch for a few seconds.

I thought it might have been something like a load of debris, but then it happened again. I stopped and had a peer under the rear and couldn't see anything - then it made a not too dissimilar sound again - at a standstill with the engine off! (I suppose it must be something expanding/contracting.)

On the 10 min drive home I didn't hear the sound again.

Car's just come back from a 2-yr service and full bolt check and only has 1800m on the clock.

Any ideas what it could be?

Thanks

Rich