In the plant room, something was screaming. It was a really loud whistle, so I looked in to see what the problem might be. It was so loud, I couldn't tell where it was coming from, but the big box that had four ventilation pipes was an blinking orange error message. I powered that off; the whistle continued, so I powered it on again. So maybe the main boiler, which was also showing an error message? I powered that off; the whistle continued, so I powered it on again.
But what about the UPS? I have a UPS in there, which used to power the fiber router (this has now been replaced by a newer UPS). But I hadn't unplugged and powered off the UPS, and sure enough, when I powered it off, the screaming stopped.