I am having similar problem on my 2016 HTRAC Genesis. 39k miles. Purchased as CPO. (100k drivetrain warranty intact + CPO Wrap)
1. Started with the fluttering noise below center console. More noticeable when cold.
2. Dealer replaced the driveshaft. Immediately after the replacement, they replaced it again saying the first one had a defect.
3. Noise went away and it drove smoothly for literally a few days.
4. The noise did not returned but had been replaced by a "shuddering" vibration right between 28-30mph. It also seems to vibrate right around 15mph but is less noticeable.
5. Replaced tires (Conti DWS 06). Alignment done. Did not help with vibration.
6. New issue appeared where transmission did not want to downshift at low speeds.
7. Dealer replaced the transmission. (Engine has to be taken out on HTRAC to replace transmission).
8. Shifting issue seemed to be resolved where it would downshift more readily at low speed/part throttle.
9. Vibration between 28-30mph still present.
10. Dealer called today saying the car needs a new transfer case for the shuddering vibration.
Supposed to be ready this coming Monday and praying that it fixes it, although not that optimistic based on others who've gone through similar fixes.
PSA: I had to go to 3 different dealers to get the problem diagnosed, acknowledged, and fixes attempted.
Dealer 1: Said there was nothing wrong with the car (when it was making the fluttering noise and even showed them a recording with it making it).
Dealer 2: Stated they were aware of the problem of the fluttering noise (had 3 in the shop for the same thing) and replaced the driveshaft. Went back to them for the vibrations and shifting problems, but they said there was nothing wrong.
Dealer 3: Diagnosed the the problem as transmission and replaced that. Went back for the vibration, which they diagnosed as the transfer case, which is being replaced.
If one dealer says can't duplicate/no problem, take it to another dealer..then another if necessary. Ridiculous there is such a variance but keep at it until it gets addressed.
What does the fluttering noise sound like?
Someone uploaded a video: