Not sure if this is what's causing the rattling, but I have found that there was quiite a bit of play in one of the front discs in the axis of rotation.
My immediate thoughts were then that either there was something wrong with the brakes or something had worn out.
Went up to the dealer first thing this morning to ask if the play was dangerous and their mechanic kindly inspected it for me. He took out one of the disc mounting bolts and I could see that the shim/spacer piece had flat spots (this was what was causing the play). He seemed to think that it wasn't dangerously so, and as the disc is pretty thin, the best solution seems to be to get a new disc, with shim/spacers when service time comes around (in about 5k (3000 miles)).
In my slight anxiety about the disc, I didn't ask about the noise, it seems likely that it could be caused by flatspots on the spacers.
If the noise is still there after the new front disc has been fitted, I'll take a look at the speedo idea.
Thanks for your help.
David