I found this on another forum A simple beep or a complex tone?
If everything seems to be working, and the sound you here is more like a siren than just a beep, the PC Health monitor on the motherboard has detected a problem.
It could be a fan spinning too slow or not at all.
A bad CPU temperature sensor.
Or a failed voltage regulator.
If you enter BIOS setup, and go to the PC health section, the source of the problem should be obvious.
You may need to replace a fan, or the motherboard