Heatshield type noise (but could it be a gasket)?
 

I've got the car booked in for Wednesday afternoon at the dealers to have a look (and of course I don't want to be misled, not saying they would) so I thought I'd ask here first incase anyone has come across this?

Ok- it sounds exactly like a lose heatshield. I've had this previously on a car. I can't remember this next bit though- when the car warms up/is fully warm the noise goes away.

So- rev from cold- rattles/oscillates. Rev when fully warmed up (and driving along) = quiet.

But...
In addition over the last few coldstarts the revs have sat at 1,500rpm. Which is higher than it usually sits at.

This is making me think- could it be a lose gasket? hole in a exhaust join/gasket?

Edit- if it is the heatshield where does it tend to be and is it something that I can tighten? (Car is a 57plate Forester 2.0R)

Firstly, get someone to help you here. Start the car from cold, get safely underneath it and get yer help to rev the car and locate where the noise is coming from. There is allot of places where noises can come from and that's the best way to confirm fault. Good luck