Why is there an engine knock after removing the bolts from the engine head?

[notACoolKid]

Hi all,

I’m new to these forums and to cars generally, so I don’t really know what I’m doing all that much but I’m trying to learn. Today I wanted to check the timing chain on my Yaris as some people were telling me it was coming loose, so I tried. I unbolted the engine head but for some reason it wouldn’t come off no matter how hard I tried. So after that I bolted everything back on and when I later turned my car on I now had a knocking noise.

If anybody could explain why this could have happened I would appreciate the feedback, I’m so lost as to what it could be from just undoing a few screws.