When my car wouldnt start after an engine swap, it turned out to the be the PTU (power transistor unit) which is basically what Nissan calls the ignition control module. And mine sounded like a "machine gun" too when I would try to start it. But when I replaced the PTU it fired right up like some sort of miracle. Now keep in mind that there are two types of PTU cause there was a recall for the first kind Nissan made. So make sure to get the newer one and the adapting piece for it if you happen to have the old one right now.