weird new problem-exhaust leak?

my car had been running great ever since i got my tweecer earlier this summer, until the other day. i had the car at about 6000rpm in second (which should be ok because i've gone this high many times before, including 6 times on the dyno, and i have upgraded springs made to handle high rpm) when the car suddenly started making extra noise. it sounded like an exhaust leak. i immediately pulled the car over and took a look under the hood. the car sounded perfectly normal at idle, but sounded like i was running w/o a muffler on the passenger side at half throttle or higher. the car runs perfectly still and there doesn't seem to be much of any loss of power. i've been driving the car since then, and nothing has changed for better or worse. yesterday, i pulled the exhuast gasket out, and to my disappointment, it was fine. i put it back on, bolted up the exhaust manifold, and it still runs the same.

when the motor is cold, i cannot feel rockers hitting the valve cover or anything. also, there is no extra noise when cruising on the freeway. it only gets loud when i get on it. like i said, it sounds as if there is no muffler. on the passenger side. i can't see any exhaust leak anywere or pinpoint the source of the extra noise, either.

anyone have any ideas or similar experiences?

thanks

good guess! i went out to wash the car and try to figure out the problem before you posted. i washed the car, jacked it up, and started looking for leaks. turns out the emissions pipe that comes out the h-pipe was not plugged. i guess the high pressure at 6000 rpm was enough to push out the 3-year old plug (bolt and rtv). problem solved. car sounds great again, and now it's clean