I'd be willing to bet that is your TOB. It is fairly easy to change, but requires you to drop the transmission. The noise is usually not a sign of anything bad, so if you can handle the "embarrassment" of the rattle I would just leave it alone. Not to mention, sometimes a brand new bearing will also make noise. It's luck of the draw.

If you do decide to change it, you need to drop the tranny, pull off the fork, slide off the old TOB, and slide on the new one. Make sure you slide it on and don't just sit it over the fork as this will cause the retaining clips to break. Very common error. I would not go to a shop for this as they will charge you a good bit in labor.

Oh, and changing out the transmission fluid, although never a bad idea, will likely have no effect on the rattle.

1999 Eclipse GS
Built motor
Hahn 16G Turbo w/ FMIC
Hahn Stage 5 Portfueler (270's & 500's)
Zeitronix Zt-2 wideband w/ 3.5bar MAP
3" SS Turbo Back
CM Stage 4 Clutch and Fidanza Flywheel
Baer Big Brake Kit