Good point re inverter, which is not infrequently associated with hum. As it happens, the inverter sits underneath the optical drive, to the right of the hard drive, near the top right side of the screen (see step 17-20 of
this guide). I figured that the hum might be resonance from a vibrating object, and first considered vibration sources closer to the area where the hum appeared strongest. Hence that fan/heat sink.
