It is in a kid book. It says, "thunder rumbled". I wonder if it is ok as it is, or the article "the" should be mandatory for the sake of the story.
Thunder rumbled is correct. If you are talking about specific thunder, then you would use The.
Because thunder is an uncountable noun, it does not need a prefix.
Countable, not a specific cat.
A cat meowed.
Countable, a specific cat (The cat because we are talking about the same cat as the previous sentence):
He saw a cat. The cat meowed.
Uncountable, not specific thunder:
Uncountable, specific thunder (The thunder because we are talking about the same thunder as the previous sentence):
There was a crash of thunder above his head. The thunder was scary.
If there is a storm or similar already, you might use the thunder to talk about the thunder of that storm:
There was a terrible storm. The thunder was the loudest he had ever heard.
This means The thunder [of the terrible storm].