OK, just thought of something else to avoid all this (don't know if it's possible though).
Could I flash my card with the Asus 1080ti Strix vbios?
That would allow me to control both my card's fan and pump speed based on the GPU's temp (both are connected to my mobo's fan headers) within AI suite.

So I have a 1080 Ti FTW3 Hybrid. Since day one it has always been silent, even with half of my case fans turned off when temps are low enough. Today I added a fan and moved another fan to a different spot. I was extremely careful with the case and didn't drop or jolt anything on or near the 1080 ti. The settings were unchanged and the radiator fans are set up exactly as they were before I noticed the noise.
 
With the additional fans, I have a total of thirteen fans in my case, and even with all of them spinning, I still hear the pump hum over the top of all of them, with or without the case side panel shut or open. You can trace the vibration making the noise. I have systematically disabled fans, including the one I just put in, and it is not any of the fans making this noise. My hearing is most definitely NOT just suddenly better than it was yesterday or last week. This is a new hum, and you can straight up feel the vibration when you touch the card. It was NOT doing this before. I am afraid I'm going to have to RMA this card, much to my chagrin. I will give it a little time and wait and see if it's just pump break-in or something. I've had it for about two or three weeks, if memory serves... "Break in" was suggested by the EVGA tech I called. How long does this break in period last for these pumps?

bryanwilson437
In AI Suite/Fan Xpert you can change it to dc by selecting the header you want to control and changing it from smart mode to fixed rpm mode.

Thanks for the reply Bryan!
 
I ended up connecting both pumps via a Y splitter pwm cable to my mobo's AIO header and set a custom fan curve.
The solution you propose would mean that I would have to raise the rpm's every time before gaming to keep the card cool.
Can't post pics or links here due to the forum's post limit but up until 45C both pumps operate at 70% and are inaudible.
They only kick in when I game and the CPU goes over 45C (the PC is solely for gaming - I work on a mac).
I found that 45C covers light usage in windows (for my case setup with Silent Wings 3 fans everywhere in a Fractal Meshify C) with me hearing the pumps.

gigah
Flashing the card with the Asus BIOS seems handy if it gives you software control over the pump and radiator, but does this affect the warranty?

As long as it's the slave bios.. don't touch the master bios and your fine.

Hmm, the 1080Ti SC2 Hybrid only has 1 BIOS chip as far as I can tell. Are you both using the 1080Ti FTW Hybrid?