Steady background noise (whistling)

Hello,
maybe a week or so ago, our Netatmo “Smart Door Bell” started adding a whistling sound to the actual audio being recorded; I can send a small video file as example. I did disconnect it from power and reconnect, but that didn’t fix it.

Last year, I had a similar issue, but it was a buzzing that went away when there was true sound, e.g. someone talking. This time, the whistling sound persists even when someone is talking. Can you please assist us in removing it?

Firmware: 2.26.0
Serial #: 70:ee:50:7c:48:2a

Finally, I lost the triangular wrench for opening the doorbell housing. Is it possible to get one or two from you?

Thank you!
Jan Charvat