I've recently noticed that I'm getting some popping sounds from the exhaust which I believe is backfiring. It happens mostly on downshifting, or when I rev the engine at idle . I've only made the most basic of modifications, CAI, SCT XCALII tune, Borla Cat Back, and had these mods for almost a year with no audible signs of backfire until very recently. Could this be attributed to taking out some timing @ the 4000-6000 RPM range to get rid of some minor pinging(something I did several months ago). What possible causes are there for this? Is it harmful to the car? How do I get rid of it? Any comments or suggestions would be greatly appreciated!