Is could you hear the nosie of waterfall a sentence or fragment?

That is a fragment. It lacks a subject (who or what is doing the hearing) and a verb.

Here are some ways to make it a complete sentence:

* "Could you hear the noise of the waterfall?" (This is an interrogative sentence, a question)

* "I could hear the noise of the waterfall." (This is a declarative sentence, a statement)

* "The noise of the waterfall could be heard." (This is a passive sentence)