I just never went back to the store, because after a quick research online, I found that there are a lot of MacBook Pros with the same thing, just search "MacBook display rattle" on youtube.What did your local "genius" say when you demonstrated that at your Apple store.
Interesting. It doesn’t bother me as well, it’s not even noticeable with normal usage. I have a 2 years warranty, so if anything goes wrong I’m sure they will fix it.My 2018 mbp 15" does rattle when shaken forwards and backwards. It doesn't bother me as I never use the machine shaking it back and forth like that. But it's a good test for sure.
How long have you been using the Air?My 2018 Air does not rattle. Neither did any other Macbook I've ever owned. I'd take it in and get it fixed if I were you. Don't settle for shoddy workmanship.