The stock exhaust manifold has this little peg as a brace inside of it that eventually corrodes, and causes a rattle at certain vibration frequencies, commonly around 3000rpms... it could be that? is it a constant noise, or just at a certain point?

the mpg could be just because you could use a tune-up, wouldn't hurt..
change your oil, plugs, plug wires, etc.
mpg also depends on how you drive...

hope it was somewhat helpful.