There is a weird raspy sound coming from my exhaust at exactly 2600 rpm. It's a raspy sound and kind of sounds like its vibrating, or the sound is resonanting within the pipes if you know what I mean. I also notice that every time I change my oil the hangers on the exhaust piping right after the cat come out. The sound occurs 80% of the time, when I'm driving and even when I'm in neutral. It only happens at this specific rpm and the rest of the time it sounds fine. Does anyone know what can be causing this sound? I have a dc POS header, a greddy sp exhaust, and stock cat. Before, I thought it was the sound of unburnt gasoline coming out of the exhuast b/c my car was running extremely rich (14 mpg on a h23 ) but I recently leaned out my a/f and it's still making the sound but before it happened around 2800 rpm. Any ideas of what this could be would be greatly appreciated.