Love it. She heard a loud floorboard creak. She heard a floorboard creak loudly. . He heard the chime of a bell. He heard the bell chime. He heard a bell chime. . The "upstairs" seems to mean it has to have been intended to be a verb here.

Creak is a verb in this sentence 😀

Creak can be a noun or a verb in that sentence

In this sentence, creak is a verb.

I would say it's a verb. I could be considered a noun, if you think of "floorboard" as describing "creak." But I think very few people would interpret the sentence that way.