@SSaub 

Normally it is fine if it is a not very noticeable/ unusual humming sound. You could try a different wall socket and see if that helps. 

To check if it is normal behavior, can you take a short video clip and share to us, you may share via Google Drive, thank you. 

Can you help provide the device hardware version, SN, and MAC number on the sticker of the extender and share via private message, thanks.