Mystery two-tone alert sound

  1. Dudes
    Cupcake Feb 27, 2020

    Dudes , Feb 27, 2020 :
    Hello everyone,

    I've got a problem that's really annoying me and would appreciate any opinions I can get.

    I've got a Oneplus 6 running Oxygen OS 10.3.1

    My phone keeps playing an annoying two-tone alert sound randomly. I don't know why it's happening, where it's coming from or how to switch it off.

    When I hear the alert I immediately check notifications but I find that the last notification is a few minutes old. I don't have any missed calls at that point. I can't figure it out.

    Help, please!