i put mine up in the air after i had the same rattle @ 3k.
found out it was a bracket that attached the downpipe (or resonator, can't remember which) to the body that was just vibrating funny. i just
bent the bracket around a bit where it touched the pipe and it stopped
i had replaced my exhaust manifold before i discovered
the solution with a known good
tii mani thinking it would solve the problem. oh well, good reason to buy parts!