Starting at ~13:05 PST today, Spotify's stream service to my Chromecast Audio devices started emitting a bizarre audible artifact when Spotify playback is paused, and between tracks when playing albums or playlists.  The artifact is a knocking sound of about 700Hz, cycling twice about 300ms apart and repeating the cycle every second.  It is f'ing maddening.

All other audio sources through my Chromecast devices a just fine - no such problem.  I've left scorched earth with all the isolation steps

After 2 hours of useless troubleshooting with Spotify '1st tier' support the problem still persists.  Listen for yourself - audio file attached.