Thanks for the reply minioneblue. The techie person at the dealers said they'd never heard that noise before!
I've now booked it in for Friday morning so they can give it a good check over but the noise now becomes more frequent as it warms up. I've got a horrible feeling something is making it under-rev as it dips to almost 600/500 revs (with air-con off). It's not even making the car underpowered or effecting the performance in any way. Bit weird.
I'll post the outcome.
