A local shop pulled my intake manifold and replaced it with the one I pulled out of another X5. Note that the manifold I pulled included all the fuel injectors. During the replacement process they noticed that the crank case ventilation valve was clogged. So they cleaned it out. The car started fine and now runs great. It turns out there wasn't any exhaust problems, all the noise was coming from the intake. So the question is, can a clogged ventilation valve cause the backfire or was the problem fixed by replacing all the fuel injectors? I have the original manifold and I'm going to check out the injectors.