I also don't have any rattles other than the privacy cover bar in the trunk (11 hatch) and my things in the door tray. Soda cans also produce a weird rattle in the center cup holders.
The WRX is poorly dampened at higher speeds and can bottom out at times over uneven freeway surfaces so you may need to upgrade your suspension if you have neck/back issues.
I've been researching on upgrading my suspension to the Gtworx setup + FR sways and bushings. While researching, I noticed many STI owners are also doing similar suspension upgrades and wonder how different the stock STI suspension/ handling is in everyday conditions to the WRX's.