I've been using phase 5/sigma slik stik stems for ages and here is the low down on fits.
Different coloured tops are slightly different sizes probably due to different molds being used.
I've used the black, the white, and the reds. The black and reds seem much looser and cause flight rattle as you call it.
However the white seems to be the most snug for me and works best with unicorn flights of any variety. Non-unicorn brand flights dont fit so well, and some especially slims are far too big to fit at all
I have noticed however the more sharply the front or back on the flight the more difficult it is to initially get it to rest correctly vs pears which I find to be the easiest.
I use white tops and find them paired with unicorn flights to be the best for a tight fit.
Also if you want flatter landing darts toss your standards and switch to smaller flights. Don't believe me? Grab some cheapy used flights and take the scissors to them!