Bingo. Here's a possible cause.
It may not solve your problem, but please test it and report back if it solves your problem.
Coil Whine Does Not Need To Be Fully Audible To Create Headaches
Even when coil whine become so quiet (threshold of hearing), it can produce nausea. You did say you get less nausea at lower refresh rates, but that it's still there. It can modulate louder and quieter, especially during refresh rate changes, but it's still there, tickling your minimum whisper hearing threshold so you can't hear the sound, but it's a nauseating audio frequency for some.
Test #1: Spin Your Computer Chair 180 Degrees
Sit in completely opposite direction staring at real world 180 degrees away from the monitor, with the monitor only 1 feet behind your head.
Sometimes nausea only occurs with your eyes open staring at the real world (even if not staring at the monitor), so closing eyes while facing screen does not always work and may create wild goose chases of other monitor-nausea causes (bright flicker or blue light leaking through closed eyelids)
If you are still getting nausea even while staring away from monitor (with monitor 1 feet behind your head) -- then there is a VERY high probability of nausea from the near-ultrasonic noises of some monitor (coil whine). Some monitors makes a lot more than others.
Test #2: Glass-Insulate Your Monitor
Once you confirmed Test #1, you can progress to an additional verification test (seeing monitor) may be needed, which requires a bit more creative testing because not everyone has a glass box to test with:
- Plywood Box with Plexiglass Front
If you are a good DIY builder in a woodwork lab, this is obvious. Be creative! Don't forget to add cooling mechanisms since a closed box can overheat a monitor (cooling ideas such as a fan hole on the side, but you also have to find a way to sound-insulate the cooling fan holes!)
- Aquarium Tank As Monitor Noise Shield
Test your monitor in a glass box. A large aquarium tank put over the monitor, is one method of blocking most of coil whine. Purchase a sufficiently big aquarium from Amazon and briefly test with it upsidedown on the monitor (easiest) or create a custom lid if keeping tank right-side-up (harder). Thicker the glass, the better. Make sure to put the monitor on a rubber mat or mousemat-style thing, to prevent coil-whine transmission to the desk surfaces). Seal up gaps caused by power/DisplayPort cables. Once you've determined an aquarium tank solves your headache problem, you can turn the aquarium tank permanent. The aquarium box can be a permanent coil whine noise shield, as long as you find a way to ventilate the box; 50-100 watts of heat builds up really rapidly inside a glass box.
- Opposite Side of a Window
- Try putting your monitor at a house window, and going outdoors to try to view your computer monitor through your window.
- If you have it in an office with glass walls somewhere, try that too (bring your monitor to work; some conference rooms have glass walls.
- If you have a car, try putting your monitor on an outdoor driveway table/cart and facing it to a side car window. Sit inside your car while staring at monitor with all windows closed and outdoor vents sealed off (internal-circulate, like you usually use when highway-passing a soot-spewing diesel semi trailer), to make your car a noiseproof box.
- If you have a French glass door, try putting on opposite side of door and seal the door cracks
1. Be VERY CAREFUL of glass aquarium tanks, they can easily shatter, and sharp glass can be killer knives. But you knew that.
2. Be VERY CAREFUL when transporting the monitor, we disclaim responsibility for a broken monitor while carrying it around.
Some bad coil whine may break through, but it will be much less. Coil whine is a major nausea problem for some people. I've chased this ghost already; It's a frequent red herring & wild goose chase misappropriating TN defects and refresh rates, for mudane issues such as near-ultrasonic sounds emitted from monitors.
Use external speakers, since you no longer can use monitor internal speakers anymore, when you're sound-insulating your monitor from problematic coil whine.
Not All Monitors Have Bad Coil Whine
You may also try monitor lottery (RMA or switch brands) until you find one with little to no coil whine. But if you're in love with a specific screen, noise-insulating your monitor is a perfectly legitimate option.
Also, not everybody gets nausea from near-ultrasonic sounds such as coil whine. However, it is part of the troubleshooting box to avoid wild choose chases to visual red herrings (visual nauseas related to flicker / blur / stroboscopics / blue light / etc) since audio headaches is part of monitor ergonomics.