Just went to Dandy Exhausts today and they reckon it's because the resonator isn't big enough to handle the resonance. They quoted me $120 to put a much larger resonator on which will solve this problem.

How can I check whether my current resonator is ok or not? I mean I can't hear any rattling when it's free revving.

If it's the cat problem you're referring to bights then does that mean my cat is stuffed? I paid quite a lot of money for this thing!