As previously mentioned in this thread, there is an issue with the ACX fans that is being largely ignored by EVGA.  The fans will spike, anywhere from 500-2k RPM at various loads and temperatures.  
 
I have tested 6 different cards on two different power supplies/motherboards and the issue persists.  The two sample images below show the behavior with the default fan curve and with the fan set to 100%.  The magnitude of the spikes increase as the base fan speed increases. 
 
http://imgur.com/qqQQ6ur
http://imgur.com/UKiABNc
 
Before I start RMAing or opting for a different model can someone at least comment officially if this is something that needs to be directly addressed by Nvidia in a driver update, like the FE fix, or if this is a fault with the ACX cooler that can be fixed with a BIOS update?   

I cant see how this can be something NVdia can fix in a driver. The input to the fan (percent curve) looks steady and the fan speed output looks wierd, so as a electronic engineer I can only conclude something most be wrong with the ACX cooler.

You said you've tried the cards on two different motherboards: was it a separate machine with its own OS/driver installation, or did you swap boards and use the same system?
 
For the record, I ran my 1080 SC on air for around two months before putting an EK block on it, and I never had anything like the fan wierdness you're seeing. The fan speed tracked the fan curve very smoothly. The fact that you've tried six different cards in your machine and saw it (with all six?) means either there's a whole batch of cards out there with fan trouble, or there's something about your machine that's driving this.

Also, the second image shows spikes up to 6k RPM. I doubt the fans can actually spin that fast, but I could be wrong. 

The Founders Edition cards had the same issue and it was fixed with a driver update back in June.  I don't know if it has the same specific cause, but the symptoms are very similar.
 
The cards were tested on two completely separate machines with clean bench installs, in addition to the other person who posted about it a week ago that has different components.
 
Yeah I wouldn't think that the fans could hit over 5k, but they're definitely spiking in between with audible differences.  And I just did another round of general gameplay testing on the two newest cards with the same results.
 
http://imgur.com/VQ3opfH
 
 

Good luck, I hope a driver update fixes it. I should be getting my 1080 SC pretty soon. Hopefully it's not a widespread issue.

Hi Dr.Doofy,
 
I am sorry to see that your card is not working as it should.  I assure you that this is not a issue with the ACX fans and just simply a problem with your product and we are here to help.  Please call or email us at your convenience so that we may work with you to have your product exchanged.
 
1-888-881-3842 Option 1
Support@EVGA.com 
 

Dr.Doofy
As previously mentioned in this thread, there is an issue with the ACX fans that is being largely ignored by EVGA.  The fans will spike, anywhere from 500-2k RPM at various loads and temperatures.  
 
I have tested 6 different cards on two different power supplies/motherboards and the issue persists.  The two sample images below show the behavior with the default fan curve and with the fan set to 100%.  The magnitude of the spikes increase as the base fan speed increases. 
 
http://imgur.com/qqQQ6ur
http://imgur.com/UKiABNc
 
Before I start RMAing or opting for a different model can someone at least comment officially if this is something that needs to be directly addressed by Nvidia in a driver update, like the FE fix, or if this is a fault with the ACX cooler that can be fixed with a BIOS update?   

Support told me last week to get replacements through the vendor, Amazon in this case, since it has been less than 30 days. The Amazon replacements I have received exhibit the issue as well, 6 in total (so far). Perhaps there was just a bad batch sent to Amazon.  
 
I will try one replacement from EVGA direct and go from there.
 
 

Brief follow-up to this issue, in case it happens to anyone else, all six cards were in fact faulty and required RMA/replacements.  There was apparently a batch of cards, sourced through Amazon in my case, that had this particular fan issue.

I have the same problem, I am in touch with EVGA support, hopefully there might be a solution via a VBIOS update or something
youtube.com/watch?v=I9KSrL8duII