I actually intended to post this to Reddit but I thought I would contribute content to here instead to get the ball rolling here and do my part.
Anyway, this is a Windows XP-era machine I have at work for testing, and I had just this monitor plugged into it and saw the CPU fan trying to spin. I spun it a bit myself and it just kept going. I disconnected the HDMI cable and it stopped.
The monitor is actually DisplayPort, with a passive adapter to HDMI which then goes to the HDMI cable connected to this PC. The GPU is just PCI-E. The computer has some old ~2007 AMD CPU in it. The GPU actually doesn’t seem to work anyway, the PC posts normally but there’s no image from either the GPU or onboard, but when putting either another GPU or no GPU, there’s an image from the appropriate output.
Could it be something like CPU generated just enough heat to be passively cooled and having hdmi plugged in causes use of pci and the CPU heats up ever so slightly to trigger the need for cooling?
The PC isn’t connected to power though, so somehow the fan has to be powered from the HDMI cable…