If the cassette joint is mounted correctly (see e.g. page 7 in this Shimano manual: https://si.shimano.com/en/pdfs/sm/IHG-INTER7/SM-IHG-INTER7-003-ENG.pdf for installation instructions), and if cable tension is correct (see pages 27-29 in the same manual) it is most likely a manufacturing defect or assembly defect. Chain tension wouldn't normally do that unless the chain is so tight it actually bends something (but if you've ridden on this bike, a very tight chain will just wear out and wear the sprocket very quickly, within a hundred km, so this sound would disappear).
From the sound of it in your linked video - one possibility is that planet gears are not aligned correctly - at least on some models, they are in fact slightly asymmetrical by design. They are the three gearcogs mounted into Carrier Unit 1 and Carrier Unit 2 in the picture on page 49 in the manual above. This manual for some reason does not mention the asymmetry. On some models, there are markings (little dots) visible when looking at the Carrier Unit from the top (or bottom), usually, the little cogs when assembling should be aligned so that the dots are pointing exactly away from the axle. Also, for some models, Shimano manufactures a special alignment tool (https://bike.shimano.com/en-EU/product/service-upgradeparts/shimano/WP-Y70800300.html). But I've once done disassembly and reassembly on an older 7-speed model, where there were neither markings nor a tool available, that was trial and error (trying different positions and listening to noise, aiming to achieve as smooth rotation as possible).
Another possibility is that for some reason, there is not enough grease in the hub (though given the irregular sound, it is not likely to be the only issue). That would be fixable by regreasing. "Simple" reoiling procedure is shown on pages 36-37 of the Shimano manual linked above. A better but more pain-in-the-neck option is partial disassembly of the internal hub assembly and greasing gears with white grease, with subsequent dipping of the whole thing in oil. Alternatively, as I do it with my own hubs, disassemble the carrier unit into parts listed on page 49 fo the manual, then wash parts in degreaser like kerosene, then blast them dry with air from air compressor, then dip individual parts of the assembly into oil, then grease gear cogs with white grease, then reassemble - I noticed that this prevents white grease from being washed out immediately into the dipping container; you can add some oil into the hub shell afterwards.
Check out e.g. this video if you are curious what the insides of a hub like that look like:
Some Shimano manuals like the one I linked above have somewhat detailed assembly instructions - if you know/can find your model number, you can try locating the manual or exploded view diagram at https://si.shimano.com/ specifically for your model. Model number is sometimes etched on the flat part of the axle - but unfortunately, some of these hubs just don't list the model number anywhere on the hub itself, so it's a guessing game. (Note: model number on the hub shell - like "SG-8R20" in the video above - can be different than the model of the internal assembly, some of these internal assemblies are interchangeable for the same hub; some are only coded with Shimano code number, the one that looks like Y-something-something, others have also their own model number that looks like SG-something; it's a bit of a mess.) Also Sheldon Brown's page (https://www.sheldonbrown.com/nexus7.shtml) lists some manuals and some homemade disassembly and reassembly instructions together with official ones. And there are several YouTube videos as well. For example, this video is very detailed (narration is in Russian but the guy shows very nicely exactly what he is doing and how, so much is understandable without narration:
Bottom line: take your hub to the dealer/workshop and demand warranty replacement/repair. Or if that for some reason is impossible, you can attempt opening and fixing it yourself, but that's pretty major maintenance work.