New build computer sounds alarm during load tests.

[RingSteel]

I just finished a new build, and I'm getting an alarm sound (constant uninterrupted beep 10-15 seconds long) anytime I load the system.

All new parts:
MB is Gigabyte GA-990FXA-UD3 (rev. 4)
AMD FX-8350 8 core 4.0 GHz Black Edition (cooled by Cooler Master Seidon 240M)
Sapphire Radeon R9 270X 4GB video card
Thermaltake Toughpower Grand 850W power supply
Crucial MX100 256GB SSD
PNY 8GB DDR3 1866MHz

Running Win 7 Pro, fully updated.

All the temps have remained below 42C, so the alarm doesn't seem to be heat related.

Even a mild load test like the Windows Experience Score test will sound the alarm within a minute.
Prime95 or Valley benchmark trigger the alarm in less than a minute.

I've tried exchanging the RAM, with the same result.
How should I diagnose from here?

 

[popatim]

Have you gone into the bios and checked at what temps & fan rpms the system is set to alarm at?