Certainly sounds like it could be a leaky turbo seal, mine was similar a couple of years back. My MB Stealership wanted to charge me to change out the gasket even though I was still under warranty, some choice words regarding responsibility under warranty were drafted for me by my old mate @jmc which I duly put to them and they covered it. Having said that, the sound mine was making was not quite a whistle like yours, more a whine, so it could actually be something else. Hopefully someone who has experienced the same sound will see your post.
Thanks for the reply. Yea I'm hoping its not the turbo really, even though it is covered under warranty still! Its not there when the engine is cold and seems to get a little louder as the engine heats up. Its not loud but it's definitely annoying!