Hi
I just bought this car , the engine start but it has weird sound when ,when you started and when I accelerate , it's like a knock but the check engine light is off, and white smoke coming out of the muffler

White smoke is an indication that the engine is burning coolant. This usually happens when an aluminum head engine has overheated and blown the head gasket. It's also happens when there's a crack in the head (less common).

What about the knock sound when I accelerate

Knock almost always accompanies this situation.