When I start the engine it rattles loudly but goes away when I am up to speed. When I start the engine it makes the tapping/knocking sound, and gets louder when I accelerate. Once the car warms up, it goes away or can be heard faintly. Once I am driving at speed I can't hear the noise, mainly only at times of acceleration. Is this something that would be worth fixing? I have heard try to sell or trade in and replace the engine. Can this be fixed? Can I fix it?