Another start up noise thread (sorry)

I've searched and haven't came across this one. If it's be written about already can spend point me to it.

http://youtu.be/SKPkAeNv1Qc

At roughly 9 seconds into the video the moment after the engine has started there is a noise that I'm unfamiliar with. I can only describe the noise as 'like a tiny dump valve'. It's defiantly the sound of compressed air being released from somewhere. Is this normal?

I only noticed it tonight and I'm not sure if it's always been there.