I get this aswell! When it's cold, kinda like a squealing noise but as you say, clears up quite quickly but depends on day to day. If someone could confirm what it is, that'd be great because I went to Nissan dealership and they couldn't help. Also, on cold days when I start it up, the revs jump up and down around idle when I first start the car. Not sure whether it's related or not...?