This seems to be a feature of some BOSE systems. There were reports that Audi had replaced some BOSE amplifiers if the background noise is to loud but they don't seem to do this anymore. I heard this noise with my Symphony II too, maybe a little bit less. But I can hear it only wihtout the engine running. While driving the other noise overlays this.
The difference might be that the RNS-E does not turn off immeditaly, it takes up to an hour after locking the car. The Symphony turned off with ignition, and with it the BOSE amplifier...