It's exhaust burble, it means your car is more manly than it used to be. Take it as a badge of honor and learn to enjoy the sound. I'd venture to guess that it's just unburnt fuel being taken care of by the hot pipes.
I get all kinds of fun sounds from my fully open exhaust.
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Competition Clutch|Fast Intentions|Maxbore|NST|Setrab|Stillen|Tomei|Uprev| Z1
Avant Garde|SPC|Stance|Whiteline
Aerojacket|Seibon|Tanabe|TRS|TWM
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