I've had this noise on mine almost since new. As you say it only appears when travelling slow say over speed bumps, and seems to be worse on cold mornings.
It was diagnosed (out of warranty) as dry top mounts, but my local garage who I trust said, unless it gets worse or starts to affect steering geomety, then he advised to just leave it - which I did, and that was 3 years ago.
Been through 3 MOT's since and also a full laser geometry set up and all with no problems. It's just a little annoying, probably will get them sorted seomtime soon.