Learn Russian: Мы слышали какой-то странный звук.

Что-то is a pronoun meaning the noun 'something.' Here, we need a word to describe the noun звук (sound), meaning 'some' or 'some kind of.' For that, we use какой-то, which acts like an adjective and matches the noun it describes.

How do you know what ending to put on какой-то?

The -то part never changes, but the first part declines exactly like the question word какой (which, what). Here, it describes звук, which is a masculine inanimate noun in the accusative case (because it's the direct object). So, it takes the masculine inanimate accusative form: какой-то.

If странный звук is the direct object (accusative case), why do the endings look like the nominative dictionary form?

Because звук is a masculine inanimate noun (not a living person or animal). For masculine inanimate nouns, the accusative case form is exactly the same as the nominative case. The adjective странный and the pronoun какой-то follow this same rule.

Could we use the perfective verb услышали instead of слышали?

Yes! If you use услышали, it emphasizes the sudden action or result of catching the sound. Using the imperfective слышали focuses on the ongoing process or just the plain fact of hearing it, similar to 'we were hearing some strange sound.'