Fan Control
I've got an ASRock Z170 OC Formula board that works alright with 4 GTX 980s. I've beefed up several of the chassis fans that handle the cooling ok, but now they're quite loud.  I'd like to be able to spin them down when the system isn't under a heavy load.  My problem is that the fan control is handled in the bios and only has the ability to monitor the mobo or CPU temp. Since most of my heat is generated by the GPUs, ideally I'd like to be able to control the fans based on that.  The Asrock board has a Windows utility that can control the fans, but I'm in Ubuntu and I need it to be scriptable. Getting the temperatures of the GPUs isn't a problem. 

So here are my two questions:
1. Does anyone know of a way to control the chassis fans on this Asrock board from the command line in linux?

2. Would anyone recommend a hardware fan controller that I can control through linux? 

Is there another solution I'm missing?
You'll probably have better luck controlling the fan speed on the GPUs rather than the chassis fans. GTX 980 is a very low-power GPU, you shouldn't have any issues cooling it.
The vast majority of the sound is coming from the chassis fans though. If I go in the bios & turn the fans down manually, it's reasonable. I'd like to drop those down so they only spin up to full throttle when I'm actually using hashcat.

I've only had this running for a week or so, but in my testing so far each GPU is running around 80° C. Is that normal? Seems a bit hot, but it hasn't reached the shutdown temp yet. I think I saw somewhere that was 91° C.
I understand the chassis fans are what are making noise. What I'm saying is drop the chassis fans down in the bios (bios should have some feature to let it control the fan speed automatically). Then spin your GPU fans up to 100% before starting Hashcat, and drop them back down to 25% when you're done. You don't need crazy chassis fans with only 4 GPUs, the GPU fan is what matters.

No, 80C is not normal for a 980, that's 10-20 degrees higher than you should be running in a 4-GPU setup. To clarify, you won't do any short-term damage running at 80C, but that's only a 160W GPU, so your temps are a strong indication you are doing something very wrong. Maybe you're using OEM design cards, maybe you're letting the GPU BIOS control the GPU fan speed automatically, maybe you hashcat will manage your GPU fan speed for you. Whatever it is, you're doing it wrong, and focusing on the chassis fans is not the place to start looking.