theres only 1 thing i hate with CoD4 at the moment and that the never ending baddies that spawn out of no where. it happened in CoD2 and its happening again, you must charge through the levels as the enemies will just keep respawning until you move the next save point.
it doesnt allow you to actualy use tactics such as taking cover, assessing the situation, then finding your targets and taking them down.
The reason they are spawning is because to don't stay with your squad. That is a built in feature which makes it harder on the Rambo style gamers.
If you get too far away from your squad or your objectives, it will spawn in ememies. If you stay close to your squad, it won't do that.