A bug I'd never heard of . . .

This is an actual support article at the Microsoft web site:

SUMMARY
During normal operation or in Safe mode, your computer may play "Fur Elise" or "It's a Small, Small World" seemingly at random. This is an indication sent to the PC speaker from the computer's BIOS that the CPU fan is failing or has failed, or that the power supply voltages have drifted out of tolerance. This is a design feature of a detection circuit and system BIOSes developed by Award/Unicore from 1997 on.

H/T to Marilyn Traber for the link.