White noise when in night vision mode

Hi everyone, I've noticed in my brand new Doorbell 2 that whenever image switches to night vision (on Live View or recorded videos) there is a change in audio quality in they way that you can hear some white noise sound (hum)? When image is back to color mode noise I gone. I haven't noticed something similar for Doorbell 1. I've already tried hard resetting the device and was testing it on charged battery. Is it normal behavior for Doorbell 2? If not, could this be hardware issue? I can upload audio file from recording. Thanks Michal