Rode Filmmaker + Sony question

[DPC]

I've just finished filming something using a Rode Filmmaker wireless lav mic running into a Sony XLR K2M on an A7R II. Listening carefully it seem as if, in the quieter moments, the noise level rises almost as if there was some auto gain control at work (which there can't be because it's absent on the camera). Does anybody have an idea what might be happening here? The effect is slight, but disturbing, especially when you have been listening to the same tracks for a week, editing ;-)