Learn Russian: Я слышу жуткий скрип и шорох за стеной. - I hear an eerie creak and a rustle behind the wall.

You would remove the subject and verb entirely, and use the particle ни followed by the Genitive case for the nouns: Ни жуткого скрипа, ни шороха за стеной. This structure emphasizes the complete and total absence of those sounds.

Both скрип and шорох are direct objects of слышу. Why don't their endings change?

They actually are in the Accusative case! Because скрип (creak) and шорох (rustle) are masculine inanimate nouns, their Accusative forms are identical to their Nominative forms. The adjective жуткий follows this same rule. If the noun were feminine, like музыка (music), you would clearly see the case change: Я слышу жуткую музыку.

Why is the phrase за стеной in the Instrumental case here?

The preposition за takes the Instrumental case when it indicates a static location, answering the question "where?" (где?). Here, the sounds are happening in a fixed location behind the wall. If you were describing motion towards the space behind the wall (answering "where to?", куда?), you would use the Accusative case: за стену.

Is there a specific nuance to the word шорох?