05 ford focus made loud knocking/tapping noise plus found oil in with #4 spark plug?

I need help with my 05 Ford focus it started making a loud knocking or tapping noise when I went to start it, I got it home and my son took out the #4 spark plug and it was saturated in oil, I'm not sure if he cleaned out the oil and put the plug back in but he drove it to the store and back said it didn't make that noise under the hood. But, are the noise and the oil in with the spark plug all related or are they 2 separate problems, because I tried to get info from YouTube but it sounds like it could be a bunch of different things, I can't afford to take it to a mechanic to have it fixed and I really can't afford a New motor or buying another car. So I'm hoping this can be an easy fix that my son can do. Thank you in advance for your answers.

Read more: 05 ford focus made loud knocking/tapping noise plus found oil in with #4 spark plug?