The power supply is indeed located in the top left corner and over the years power supplies have often exhibited buzzing sounds when they are failing. The LED driver board is also in that area though it doesn't have nearly as much power flowing through it so buzzing would be kind of odd. With what you've given us I'd say the power supply is probably a safe bet.
With that said, if it fails again I would check the diagnostic LEDs to see where the power flow is failing. Most of the 27" iMacs (before they lost the optical drive) have 4 diagnostic LEDs just to the left of the RAM slots, visible through the grill at the bottom of the Mac.
LED 1 indicates trickle power is present, this should always be lit if the Mac is connected to a power source.
LED 2 indicates that the logic board has received power from the power supply. This is the one I'd expect to see be either noticeably dimmer or dark compared to the first one if the power supply is flaky.
LED 3 indicates the video card is functioning.
LED 4 indicates that the display is functioning.
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