Do you have a HKS 4-1 manifold?
Check this thread - http://www.tezzaworld.com/viewtopic.php?f=28&t=9965
- if your noise is the same as the guy on the other forum its deffo the HKS manifold. That is exactly what mine sounded like and it took me forever to pinpoint the issue (its a plate inside the 4-1 part of the exhaust that gets loose btw).
Also I'm not a member at the other forum so if you could point the OP in the direction of the above link I suspect it will help him out (he states he has the HKS).