Yea man.. my ej20 has that same rattling sound.. somtimes it will rattle.. and sometimes it wont.. have you checked all the motor mounts.. maybe one is broken???? if you find anything out... let me know so i wont have any future damage

 

My 2005 has the same issue...weird rattling around 4k rpms in gear

 

A throwout bearing only rattles with the clutch engaged (pushed in) and a bit when you first let out and accelerate. Bad cases you can hear the rattling all the time, but his usually means the bearing is shot and you can't shift anyway. EJ20...you might want to check yours. Not uncommon in the WRX (older models that is).

For a rattle noise at a constant RPM in a specific gear only after a period of driving....good luck diagnosing this. Definitley do a fluid change as it's always a good idea anyways. Could be just normal for the car too. You can never tell with an STi as far as noises go. Could be: Tranny, cluthc, front diff, center diff, rear diff, wheel bearing, and basically, anything in the drivtrain. If it were my car, I wouldn't be too concerened EXCEPT for the fact that it appears only when you been driving it for a bit. This would indicate something associated with stress or temperature, and could be a precurser to worse things to come. If you are under warranty, def have the dealer check it out. Schedule it before hand though so that you have the car warmed up and making the noise, then pop in and have the tech jump in so they can hear it too. Don't let them try to replicate the sound as 1) why give them the opportunity to abuse the car and 2) I've never been successful in having a dealer replicate or diagnose any rattle.....EVER.

Good luck and let us know.

PS....I highly doubt it is the motor mounts, but stranger things have happened.