2001 Alero Engine Knock..

Hi, i own a 2001 Olds Alero, 2.4, 4 cylinders, with only 100k miles on it, And unfortunatly, lately i've had a rattling noise in my engine when i reach about 3000rpms, and then it comes and goes, although my oil is somewhat black, it is 3/4 full, and i do plan on getting it changed very soon. Any ideas on what it could be? :banghead:

Without a sound clip we would just be speculating. And even then, knocks are hard to diagnose.