There's a server that I use purely to givev me multiple windows on to various servers, and that server started singing to me today.
It's done that before, and I know what it means - the CPU is overheating. Which probably means that the CPU fan has stopped working.
That's annoying. I replaced that fan only a few months ago.
So, without powering the server down, I took the cover off, and sure enough, the CPU fan wasn't spinning. I removed it and replaced it with a similar fan. There was another fan blowing cool air into the server that had also stopped, and I replaced that too. I put the cover back on, and now it's humming almost silently; no annoying singing.