Why is my engine making a tapping/knocking sound? on 2007 Mazda Mazda6

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?

Asked by for the 2007 Mazda Mazda6
START BY CK'ING OIL LEVEL.HAVE YOU DONE PROPER OIL CHANGES IN THE PAST OR HAVE YOU GONE EXCESSIVE MILES WITHOUT PROPER MAINT
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