Is thundering a pronoun

No, the Word 'thundering' is the present participle, present tense of the verb to thunder. The present participle of the verb also functions as an adjective and a gerund, a verbal noun.

Examples:

The cloud of dust was created by a herd thundering across the plain. (verb)

The thundering footsteps on the stairs announced the intermission. (adjective)

We herd the thundering of a storm in the distance. (noun)

A pronoun is a Word that takes the place of a noun in a sentence.

Example: We herd the thundering of a storm in the distance. It seemed to be moving closer. (the pronoun 'it' takes the place of the noun 'thundering' in the second sentence)