Pixel 2 repeatedly playing a sound alert without explanation, what does it mean? (Sound included)

My Pixel 2 has gone crazy in the last few days. Maybe related to the latest Android update that I got a few days ago, but I'm not sure. I recognize the sound (I have heard it earlier) but it has never behaved like this before. When the screen is on, except in phone calls, it will (randomly?) play this sound over and over, sometimes 20 times in a minute, sometimes it will be quiet for a while. I haven't figured out any logic of when it plays or not, but it stops if the screen is turned off, and it stops during phone calls. It also plays when using other apps, not just when showing the desktop. I'm attaching a sound sample here, 28 second recording on the phone itself just lying still. https://drive.google.com/open?id=1tQUkt1pUj9yFRGNs6jDoFxsVj7HYnpP8