Indeed, as Adroid said it's commonly... for a few PSU's from a certain batch. You can say 2 out of 10 PSU's have it or will get it from the day of purchase or later.
Most people however, think it's damaged parts/damaging stuff, but it isn't. The PSU will have the same lifespan as an other not having it, but the noise can work onto your nerves and you'll better RMA it. Even if you have no receipt they would just read serial/brand info and for their imago, fix it or return a new one.. For me atleast.