it could also be from your cam timing if you have adj. cam gears..my best friend has some on his car..we retarded the exhaust cam gear 2 deg. and it fixed it..it could also be from a free flowing exhaust...its just the exhaust gasses going through the exhaust..as long as its not popping like a backfire..i wouldnt really worry about it..you can check the spark plugs and see if its running rich..if it is...you may want to either advance timing on your engine..or get a Fuel Pressure Regulator to help this problem