Weird fan behaviour



so I've been undervolting my CPU on the weekend and it seems to be stable at 1V. But I encountered a problem with my main CPU fan in the process of undervolting. It randomly ramps up to 100% and stays at 100% for a random amount of time (sometimes for hours) and then suddenly ramps down again to around 700-800 rpm. I didn't change anything in the BIOS other than the CPU VCore. The temps are low and there is no reason at all for the fan to ramp up.

I tried loading the default profile for the BIOS, but it didn't help and now I'm kind of clueless. Did I somehow manage to corrupt my BIOS?

Intel i5-4670
Gigabyte Z87-D3HP
G.Skill Ares 8GB DDR3-1866
Gigabyte GeForce GTX 770 WindForce 3X OC
Cooler Master G600 600W

Already searched on google quite a bit, but couldn't find anything.


Some BIOS settings:



Have you looked at the task manager when it does this to see if there is some process using the CPU?

The CPU isn't being used at all. And even if, my cooler is totally overkill. When it is not spinning at 100% and I put some load on my CPU it spins up to 1000 rpm MAX (100% is around 1800-1900 rpm).

Ok, I've tried updating the BIOS. The BIOS looks a lot better now, but it didn't fix the problem. But what seems to have fixed the problem is simply switching the OPT_CPU fan with the main CPU fan. No Idea what's that all about, but if it keeps working I'm happy.