put the car on a lift and get under it, check the shield that is attached to the pipe just before the cat-converter, its a small thin piece of metal plate that is either for blocking debris or heat (not sure what heat), my car made a similar noise under acceleration, this was because the bolt holding the front of the plate up rusted off, and once the engine reached enough RPM it caused it to vibrate. if it wiggles around then that could be your problem, if so just rip it off, cured my problem. Had a similar problem when I put a short shifter in, the long heat shield above the exhaust rubbed the shift linkage and had to be bent back a little.