Chatter under load, between 2k and 3500 rpm

[92 mtechnic cabrio]

I've tagged every heat sheild and braket connected to the exhaust with a hammer and not so much as a rattle...

doesn't have to be a heat shield

exhaust gas exiting somewhere besides the tail pipe (a leak) would make a sound similar to metal on metal.

my friend's miata has an exhaust leak on the pipe between the cat and the headers and it sounds a lot of valve tick.