First, I'd check if this isn't made by either some distortion, saturation or tape/vinyl sim plugin. Sometimes those can generate some noise on purpose. If so, gate that particular track and you're done.
If that isn't the case, I'd suspect you might have dithering active on several stages of production. May be you render it out with dithering, turn dithering on in oZone and then render mastered result with dithering again? That might add up dither noise to the point it's audible.
