Questions & Answers about Кто-то слышал странный шум?
Why use кто-то here instead of кто-нибудь, since it is a question?
Usually in questions, we use -нибудь words (like кто-нибудь for "anyone"). However, you use кто-то when you suspect that a specific someone actually did the action or heard the noise, rather than just asking about anyone in general. It makes the question sound like "Did someone hear that?" rather than "Did anyone hear that?".
Why is the verb слышал masculine if we don't know who that "someone" is?
In Russian, the base word кто (who) and its related words like кто-то (someone) are always grammatically masculine singular. Even if you are in a room full of women, you still use the masculine past tense слышал to agree with кто-то.
Why use слышал (heard) instead of слушал (listened)?
Слышать means "to hear" (a physical sense that happens to you), while слушать means "to listen" (an action you do on purpose). You wouldn't intentionally listen to a sudden, unexpected noise; you just hear it, so we use слышал.
What case is странный шум in?
It is in the accusative case because it is the direct object receiving the action of the verb (the thing being heard). Because шум (noise) is a masculine inanimate noun (not a living person or animal), its accusative form looks exactly like its basic dictionary form.