Exclamation [Q] Strange buzzing sound

Hi all,

I've discovered a problem (not sure it is a bug, faulty hw, or just a setting) in my z1 compact. Seems that everytime a sound terminate (maybe because I stopped the spotify song, the sms song ends, etc) a strange, weird, sound continues to be produced for a couple of seconds.
I know it is difficult to explain: try to imagine when you register something with a microphone, then what you're recording finish and the silence is recorded as 'zzzzzzzzzzzzzzz' like a distorsion or interference.

This is the issue. I've noted then during the night in the bedroom and when you've silence is pretty clear and audible. As is present with any sound, from internal sounds to videos and spotify, I think it might be a sw bug or a wrong setting somewhere.
Have you noted something similar or might you check?