Oooh. Cali. Sorry.
It's the uel headers providing that "boxer rumble". As many on here are fond of saying ... ah, the sound of inefficiency.
Supposedly, el headers can make slightly better power. And sound a lot more like a Honda. Look for some youtube goodness of uel vs el headers (the examples I've seen were on a wrx, but same difference in this case) and you can get a sense of it.
I'm not entirely sure why Subaru uses uel headers from the factory. Is it a fitment issue? A boxer rumble marketing thing? I'll bet it's been answered somewhere on these forums before though. Maybe some other enterprising member will search and post a link for us on this point, since I'm too lazy to do it right now.