My car has a my car has bad knocking sound had to change oil.did not stop it

What is your question?

I bought the car and the guy said the sound would stop when he changed the oil it didn't...the oil was really low and dirty...please help me I need to know what it is I thought it was the lifters but a mechanic said it sounds like a rod...didnt even look at it just listened to it..what could it be