First Place To Start, When dealing with Start up knock/hammering?

Own 2010 Prius IV . Now has 205k ....First Fall I owned this car It made a hammering sound on one start up. That was three years ago. My wife drives 100mi. one way to work, arrives, parks car , when she returns after work, she says it hammers and shakes for a full minute before quieting down.

It throws a CK ENG code of miss-fire, so far I have changed the plugs. unfortunately it did it again on me this-morning. I know this can be several things....I am just trying to narrow down the probabilities, before I spend thousands on an endless search for my fix.

Next I was going to clean EGR....Or should I replace coils? or Intake first? So many things can cause a miss-fire. Just figure there has to be a fix that is more probable than others.

Thanks

 

Thank-you very much for the input....I pulled Intake & EGR tube...Both were filthy, so I'm sure that will help something....but yea, antifreeze was about 1/2 inch low. If it does it again, it will be time to either glass it, or add some blue devil while I hunt down an engine. Although in theory, a good glass job, could fix a seep that small for years.