Desktop makes loud siren noise when I turn it on that won't stop

[Helpme1982]

When I turn on my computer, there's an extremely loud (boop boop) siren noise.
It doesn't stop. I have to unplug to get it to quit.

[mdklassen]

That's an audio alarm that the computer is overheating. Shutting it off is the correct action.

Power-off the computer.

Disconnect the AC power cord.

Remove the side-panel, to expose the motherboard.

Check that the power-leads from the fan on top of the processor are properly connected to the motherboard.

Check that the fan on top of the processor spins freely -- not stiffly, not "choked" by dust.

Check that the heat-sink on top of the processor is not "choked" by dust.

Without replacing the side-panel, reconnect the AC power cord.

Power-on the computer, and watch to see if the fan instantly spins.

If it does not, quickly disconnect the AC power cord.

Then, after removing the AC power cord, remove the fan, to expose the heat-sink.

Use a can of compressed-air to remove any dust on the blades of the heat-sink.

Take the "dead" fan to a computer store, and get help to purchase a compatible replacement.

Install the new fan.

Reconnect the AC power.

Start-up your computer.

Because a few hours have elapsed (while you went to the computer-store) It's possible that the computer is "cool", and may not instantly sound the alarm.

Feel for air behing exhausted from the power-supply.  There's a fan inside it, which also could have failed.  No "breeze" means that this fan has failed.