What is the grammatical function of the word “suddenly”:

Read the text to answer question.

We were on a flight to Tokyo, and we’d been flying for about five hours. I was reading a book, and my wife was watching a film when suddenly we heard a very loud noise. It sounded as if an engine had exploded. The pilot didn’t tell us what had happened until half an hour later.

Source: OXENDEN, C.; LATHAM-KOENIG, C. English

File Upper-Intermediate - Student's Book - Third

Edition. Oxford: Oxford University Press, 2019.