It took a little searching to solve a problem on my daughter’s windows PC. Every so often, the current window would flash with inverted colours (white to black, blue to orange). TLDR, it’s an accessibility setting (https://answers.microsoft.com/en-us/windows/forum/all/windows-10-screen-flashinginverting-colors/fbcbc715-8fd8-4476-9646-ff95739437e1)

It took a while to notice it was the window, as she runs word etc full-screen. We replaced the HDMI cable, rebooted, considered the graphics card.

Anyway, an over-active-but-unread discord channel was making notification sounds at frequent and random intervals. Both issues are now fixed, happily.