So I need a little help to understand what the right side engine cover where the timer cover is called and what is behind it.

I can hear something rattling in there and can feel something that feels like a rock when I put my hand inside it from the bottom. I'm thinking of opening it up, but I cant find any information about what is actually behind it, what it is called and also anything I should be concerned about.

I've attached a picture to make it clear what I'm talking about