I recently did a motor swap b16a2 in to a 2000 civic dx.. When at idle its not that loud, but while driving its enough to be an annoyance. At 3400 rpm (mostly 1, 2, 3rd gear) there is a rattling around the vicinity of the headers. Only at 3400, not any higher or lower, the sound jest goes away. Ive checked the header, the heatshield on the cat, and felt around for anything loose but didnt find anything. Does anyone else have a similar problem or insight to what the hell it is? Its really annoying and its pissing me off...

components:b1682, exedy stage 1 clutch, prostreet 12lbs flywheel, dc(onepiece) 4-1 header, rsr full catback exhaust, ES urethane torque mounts.