What should an engine sound like?

Come to think of It, when we started up my cousins mustang a while back it had a similar noise. But I fail to see how it could be completely quiet, there is alot of stuff going on in there even at idle.

But lets just say worst case scenario, what would a hurting cam sound like? Would I know if that really was the issue? Backfiring through the carb? because it has yet to do that, or even backfire out the tail pipe.