So I bought a 1080 Ti Hybrid today. Unfortunately, the guy who sold it to me failed to mention that it had a bad pump. Now, I hear a terribly loud constant buzzing noise from my graphics card. Thermals are perfectly normal, but I can't deal with the noise. I wanted to replace the water cooler on it (part number: 400-HY-5188-B1), but unfortunately, since the 1080 Ti is so old, I cannot find this water cooler anywhere. Can you guys suggest a substitute cooler that I could use as a replacement?

I was in similar position with my 2 980 Ti Hybrids recently. Decided to clean my PC and after putting everything back together one of the card made an awful noise and idled around 70C. Shaking did not help. Radiator was above the GPU with hoses coming out from the lower part. At one time I got it stable at around 60C (still noisy), but as soon as I moved the radiator, the temperatures went back up to 70C+. To put things into perspective, before cleaning I saw temperatures around 75-80C when playing. So quite bad for AIO, but I had accepted that temperatures got worse over years.

I did look at the replacement coolers:
* No luck finding 980 Ti Hybrid kit.
* Faulty 980 Ti's were too expensive at eBay for my taste.
* There were no good offers on Arctic Accelero Xtreme IV at eBay at the time. Didn't see it available anywhere else. Although looking at the eBay right now, some have OK price.
* RAIJINTEK Morpheus II + 2 x 120mm fans
* NZXT G12 + (semi-)compatible AIO - most of the compatible AIO coolers are not available anymore, but some of the newer coolers might fit the mount.

The last 2 options would both have cost me around 100-110€. One thing I didn't like about Morpheus II was that it would have made my GPU way too thick, 4 slots. With NZXT G12 I was thinking about using a Corsair H105 cooler I had on my CPU or buying a used Thermaltake 280mm AIO.

The last 3 options should also be compatible with 1080 Ti. Instead of NZXT G12 I was also considering 3D printing the mount (you can Google "Prototype A 3d-Printable GPU AIO Mount!"), since I didn't like the idea of removing the turbo fan from 980 Ti.

What I ended up doing was to fix the original AIO. If the actual electronics of the pump had been failing, I would not have been able to fix it. But in my case, the buzzing noise came from propeller inside the pump. For the first Hybrid I removed the copper cold plate, drained it, used a toothbrush to clean the fins, flushed the system a few times, filled it back up and reassembled it. Since I wasn't sure it's watertight, I bought a GPU fan to 4-pin fan cable so I could use the pump without the GPU. After a few attempts it still made noise, but wasn't leaking. I did try to increase the speed of the fan and pump to 100% in the BIOS using the HDMI output of the GPU without the cooler, but I guess I didn't do it long enough, because after deciding to put the cooler back on the card, the noise lasted about 30 sec, then the air ended up in the reservoir and it was silent again. And after 30 min of running Heaven benchmark the max temp was 51C . After the success I also decided to open up the other AIO. This time I didn't use toothbrush to clean the fins, because on both cases the rubber gasket that covers the fins was stuck and I didn't bother to pry it open this time. I did however use a syringe to get rid of most of the particles. I then filled it up without draining (if I remember correctly I was able to add 10-15 ml of liquid) and put it back together. And after all that trouble I finally realized there is an easier way to fill the system: there is a screw on the electronic side of the pump you can use to fill the system. For other GPU the max temp while benchmarking was 58C, so I guess proper cleaning does help. But 58C is not bad either, because I did run a benchmark on that GPU before messing with AIO and it was 84C. Replacing the thermal paste might also have helped.
 
In summary, if changing the position of the radiator or shaking the card and radiator doesn't help with the noise and if the actual cause of the noise is the pump propeller, (re)filling the AIO might solve it. The whole process was easier than I first thought and regarding the AIO, I had nothing to lose trying to fix it, since it didn't work anyway.