LOL "no mods.."
never change the exhaust on a modern car with computer controlled ignition without adjusting the timing. Of course _you_ can't adjust the timing! Now the engine is running ignition advance that's way too high for the new exhaust. I bet if you tear down that engine there will be pits all over the pistons and heads from the detonation.
2011 E90 M3 ZP2 ZCV ZCW ZCP (for teh fastAr)
2005 LGT, AVO380 Turbo, a zillion other mods, ECU tuning by me.