You sure wouldn't...because the 2021 Civic SI doesn't exist. LOLThe rattles on the 10th gen are early model year issues unfortunately. If you were to get a 2021 SI it would have absolutely none.
The only issue that remains is Honda's lack of sound deadening standard in the doors and other parts of the interior which contributes to road noise. The Accord is much better in this area and even has active noise cancellation but that car also carries a heavier price tag as a result.
Unfortunately it's common for many car brands to have rattle issues in early models because it often ends up being that "sample size" problem to have the problems show and get brought up to the manufacturer. Some brands just do a better job offering fixes at dealership service to fix them.
If you want a rattle free car, I would wait at minimum the 3rd year of the 11th and not the first two years or lease the first year model if you cant wait but I wouldn't buy it until a later year