Hi,
I'm getting a similar issue with my Y reg, only 65k on the clock. The rattle is intermittent, sounds quite tappety but not too sure on the origin, it can happen when the lump is hot or cold so I know this doesn't effect it. I had a bit of a poke around the engine bay and all covers appear to be secure. Fortunately it's booked for a full service on Monday so I'll get them to check it out then (providing it happens when it's in the shop, sod's law it won't).
Thanks for the advice on the pulleys/belts I'll have a look and get back next week if I find anything.
Only brought the car July 2010 and haven't noticed it until last month (May 2011), (Prob driving like a tw*t so didn't notice)
Cheers
B