Loud knocking noise when driving

I have removed the manifold it was really dirty and had oil in it and then moved to the fuel injectors and they had build up on the ends then moved to the throttle body cover and it was very clean and I was told it could be a blown head gasket and I would know by pulling that cover off because I would have gunk build up. Now I'm lost should I just go ahead and get down to the head gasket and change it to or what should I do now, any suggestions? In need of some help plz I'm in a rental and it's to much. 2013 gen 3 Prius

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