I hope someone figures this out...
I bought my 2018 Limited Hybrid in February, and it has made the sound that just as you described I also thought the same - I'd swear sounds like the rear hatch.
Attempting to press down the hatch - test for play results in no movement... I have had cars like my CRX and Starion whose hatches had a spring-loaded plastic wedge that would block the hatch from hitting too hard - but it also had like a 1/4" of movement - you could press it down. I didn't notice any movement in my Ioniq hatch.
So despite the sounding exactly like an loose hatch - I don't think it is. Though, I had an idea the other day - and have yet to try driving with the hatch open, perhaps secured with a strong bungie cord a couple inches from max, so it doesn't make the noise coming down on bumps and hitting max open. to see if it still makes the sound, as that would certainly answer the question if it is the hatch.
My fear, being a guy who has replaced struts on Honda's and Mitsubishi's - my second guess would be, well, I'd rather hope not - what if one of the rear strut towers didn't get tightened, and has a 1/2 inch play - that would make the same bumping sound on bumps as the car re-settled on the strut on bumps hard enough to bounce car weight off the strut - fear comes from if true - what if the threads act as a file on the holes, ripping the holes wider and destroying the mount, or worse, nuts come off, then on a big bump a wheel fails...
I called the dealership, talked with them - all they could say was 'bring it in'... like I have time to do that, and sit and wait...
Considering others have the same noise - hopefully one of us will find it. If I do - I'll share!
Love these forums - Interesting reading some of the posts... from informative to scary... Thanks!
Todd