The bottom part should hold the brass washer in place. That thing should not move around.
Wiffle, what's so dramatic? It's a spring stack, made of 8 washers. No pressure leaves the washers with some space to rattle. It is not a spring, it is a stack of washers. Nothing dramatic there, pretty much normal obvious stuff. I have no idea why you'd be confident it would not function, because of a rattle noise, since you are very familiar with sidewinders.
Btw, I use that purple pump Evolution you sold me 3 years ago as main go to pump marker. That thing is really nice. Made some magic shots with it. Got it a purple pump handle. Sidewinder si flush on it, have not adjusted the ivg many times over the years. Always shooting 290fps. And making me smile.