Quiet form of "Hey!" (4)

I believe the answer is:

psst

(Other definitions for psst that I've seen before include "Surreptitious attention-seeker" , "Sibilant sound to attract attention furtively" , "Attention-seeking noise" , "Hissing noise to attract attention" , "Beckoning sound" .)

I've seen this clue in The Washington Post.

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