Do you have a spare power supply? I've seen that bit of spinning up for a second or two on desktops, the culprit has always been the power supply was failing. Note that power supply testers would report it ok--the problem occurred only under substantial load. There's a short window in which the voltage is permitted to be out of spec, once that timer runs out a too-low voltage results in an instant shutdown. This is a safety measure, it kills the machine before it reaches the point the electronics become unreliable.

Remember in the old days how they told you to remove floppies from the drive before turning off the power? This is why--they didn't have that sort of protective circuit back then, as the power supply capacitors drained there was a narrow window where the machine could go wonky--and wonky could include things like issuing a write command to the drive.