It has been a "problem" for many years and has been aptly named..... "startup clutter"!!!!!!
Whatever it is it is harmless (noone with clatter has something bad happen to the engine for at least 7 years now) and may be reduced or fixed by switching to a lighter engine oil, something like synthetic 0W-40. Next time you change oil try Motul 300V 5W40 or 8100 Ester E-Tech 0W-40. I did and clutter is gone.
Anyway, if you do a search with the words "startup clatter SR20" in google.com or something, you will find a few hits in se-r.net where the problem is discussed.
My lifters are noisy, I have something that rattles, and my starter used to make a noise after starting. The starter noise was fixed by pulling it out, and taking it apart and liberally greasing the gears. It only made the noise once more after that, the first time I started it after having done that. Started it probably 10 times since and no noise from it. Still have noisy lifters and a heat shield or something that clatters but haven't addressed those yet.
When it's noisy on startup and goes away, it is your timing chain tensioner. Mine used to be noisy on startup. I replaced the tensioner and it is now quiet. It really isn't hard to do yourself. It's located to the right of the oil filter if your looking at it from the pass. fender. If you look on se-r.net you will see writeups and pics how to install it. Just be careful to take out the tensioner slowly. If you let it spring out you can get pieces to fall inside the engine, then you'll be pulling the oil pan. If you have any questions let me know. I can walk you through it.