fitted a complete de cat system to classic sti ra and noticed alot of noise coming from the down pipe . just sounds like the noise you would get from the engine echoing down the pipe , also on lift off after reving the engine there is this strange weazing sound like the noise a rattle snake makes , to be honest it sounds ****e , is this just me or does a full de cat system usually excentuate the engine noise down the pipe this badly.

 

Mine is the same,very noticable once i fitted the decat downpipe.... just resonance as far as i can tell as the gasses leave the turbo and find no restriction to hinder the flow as when the cat is there....... forget about it after a while.