OptiPlex internal speaker noise with computer powered on or off

I've been working on a problem for a week now. First off, I’m experienced with computers and have been using them for a long time. I have an OptiPlex 9020—though I’m not sure if that’s relevant to this issue.
For sound, I use external speakers—specifically, a pair of 50-year-old Norman Labs that perform exceptionally well for both computer audio and rock and roll. I almost never hear my internal speakers unless my cat unplugs the external ones, which automatically switches audio to the internal speakers until I reinsert the jack in the back.
About a week ago, my internal speaker started making a notification sound, similar to one of Windows’ preset minimize/maximize sounds. During my investigation, I discovered that this sound plays exactly every 60 seconds. Even stranger, it continues even when the computer is turned off—it’s making noise 24 hours a day.
While troubleshooting, I also found that my sound settings had unexpectedly switched from the standard Windows configuration (which had remained unchanged for eight years) to Dell Audio. With Copilot’s help, I managed to revert the settings—turns out, the driver had been altered. Instead of letting Windows automatically select the “best” driver, I manually chose the Realtek driver, which restored my previous sound settings. Copilot mentioned that an update likely caused this switch, but adjusting the driver didn’t resolve the recurring notification sound.
I've tried every suggestion Copilot has provided—usually, I had already attempted them before it suggested them. The only thing I haven’t tried yet is unplugging various components, which was Copilot’s last idea. Even Copilot commented on how odd this issue is!
I really appreciate Copilot, and on the off chance that some tech expert out there recognizes this problem, I figured I'd post this to see if anyone has insights