。
There was a clap of thunder.
Literal
Thunder [subject-が] thundered.
とどろく ('to roar / boom / resound') is a literary verb mostly applied to thunder, cannons, applause, or fame — partly mimetic in origin (compare ごろごろ for thunder). Note the writer's choice to use kana for the verb but kanji for the noun 雷鳴 ('thunder-sound'). A common stylistic pattern: kanji for content nouns, kana for verbs — keeps the visual flow lighter.