Overheating is, of course, the main reason for alarms beeping.
Fans moving too slow is the other.
Voltage overload is the third I can think about, yet I don't know for sure of any motherboard that has this capability.
Maybe you are missing some info in your sensors software.
I suggest for you to check sensors (temps, fans and voltages) using:
- AIDA64, if you run Windows.
- "sensors", from "lm-sensors" Debian package, if you run Linux (if someone knows about a better software, please report!).
Many sensors are only detected by highly specialised software. AIDA64 is the best, as far as I know (if someone know about a better software, please report too!):
It is not free software as for now, but you can download a fully functional 30 day trial version from their site (any version should work; I prefer Business, for their capabilty to remote management, but here you have the comparison between versions.
If possible, report what happent. It could be useful for some of us.