Why is my flash making a high pitched whine constantly?

While older flashes have a true on/off switch that physically cuts battery power to the electronics, newer flashes like the 430EX switch power on and off by electronic means. It looks like the electronic switch has shorted out, or the command circuitry has developed a problem so it leaves the electronic switch eternally on.

The whine you hear is the inverter that charges the high voltage supply (300V or so) to the flashbulb. Normally speaking the inverter should work only for a few seconds each time after the flash has been fired.

It is very unlikely you can fix this yourself. Repairing this sort of faults requires advanced knowledge of modern electronics and troubleshooting, and it might even be necessary to replace an entire circuit board - which is something that can normally be handled only by authorized service centers.

Warning: Flash circuits carry high voltages that may persist for quite some time even after batteries have been removed. DO NOT dismantle the unit unless you know exactly what you're doing!