I came across this prob with mfi, what i found out is the 5th (cold start) injector was dribbleing fuel due to the pressure in the system, which is still there after you switch the engine off, so the fuel vapour finds its way into the hot exhaust manifold and det's making the pop you hear, change the injector (for those who dont know, its the one under the inlet manifold) and it will remody the pop.