I noticed it about a week ago and can't recall hearing it before then. Likewise the car had been warmed up. I haven't changed the oil in it yet so I can't relate it to that.
Could the alternator make a noise like that?
Mine does the exact same thing. I have no idea what it could be either. My Nav has only just done 6000km.
It's due for the 6 month service, so I'll see what Nissan says.