Your computer doesn't look at your o2 sesnor for input on a cold start. It won't start looking at the o2 sensor until the car has warmed up. Could you possible be hearing an exhaust leak? Is it like a ticking poping noise, like a lawn mower? I've seen cracked exhaust manifolds before and sometimes aftermarket headers give you really thin mounting flanges and they tend to warp creating an exhaust leak.