Q: My Engine started making a small knock on 2002 Isuzu Axiom

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I let my boyfriends step-dad work on my car bc he's a mechanic. when I bought the car 3 months ago, the guy said he put a new motor in it. there's 65xxx on it. after the last time my boyfriends father messed with my car, it's caused a knocking noise. what could it be??
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Need more info. Knocking noise could be caused by low oil level or wear in the pistons. Check oil level and go to parts store and get an oil detergent additive to see if this clears up. If not, repost and describe if the knock is constant after this procedure or only when the engine is cold and warming-up. Lifters can cause this and the detergent will ease the problem.
Jesse, my boyfriends step-dad, said that it wasn't a knock, that there's a crack in the exhaust pipe by the engine that is causing the noise. I told him i hear it when i'm idling and when i'm between 25-30 mph & 40-45 mph. He said it wasn't the mph it was the rpm that was makin the noise. I had my oil checked bout last week, the guy said my oil was completely empty, but no light came on. He put 3 qts of oil in my car then it said it was full. i don't know if it helps that i had driven my car around all day and then the guy checked my oil b4 the car cooled down or not.