誰かが口笛を吹くのが聞こえた。

who

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

whistle

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to blow (e.g. wind); to play a wind instrument; to emit; to spout; to whistle; to laugh; to burst into laughter; to smelt; to mint; to brag; to talk big

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

to be heard; to be audible; to be said to be; to be reputed