The expression "suddenly" means "suddenly", "suddenly", "suddenly" and has syntactic function of adverb of tense or of manner and, therefore, it is an adverbial phrase, since it is the set formed by the preposition “of” with the noun “suddenly”.
the preposition "in" is antecedent and introductory of the noun "sudden", therefore, there is no joining, merging between the two terms and yes a connection, so that there is a complete sense. Then, the expression used of correct form is separated: “suddenly”.
Suddenly, a strange noise came through the window.
Suddenly, a strange noise came through the window.
Suddenly, a strange noise came through the window.
See an example with "suddenly" in the adverb function so:
He suddenly entered the room. (the way he got in)
Different than:
Suddenly, the boy entered the room. (related to the time the boy entered the room)
By Sabrina Vilarinho
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/de-repente-ou-derrepente.htm