hmmm...u know, my exhaust rattles around 3k rpm too. It takes approx 5 min or so of street driving for it to start rattling from a cold start. Consistent too...at the same signal light everyday on my way to school it starts rattling. only way i've gotten it to stop was to go onto the freeway to heat up the car.
I took it into the shop and the guy there told me it's possible it could be the baffle inside the center resonator broke off. If that's the case, then when I take it on the freeway or drive it alot continuously the baffle expands because of the heat and doesn't rattle anymore. U think this sounds like a similar problem u are having with your exhaust?