I have seen people using this word to refer to the sound wind makes as it moves through trees. However,
1. No reputable dictionary seems to have acknowledged this term as a valid english word.
2. Even Google Ngram seems to agree on its oddness.
It appears to be an obsolete rare term.
Fom the OED Online:
Whispering; a whispering noise, as of leaves moved by the wind.
1872 M. COLLINS Pr. Clarice II. xix. 218 Psithurism of multitudinous leaves made ghostly music.
1875 Blacksmith & Scholar (1876) II. 12 The wind wooed them with a whispering psithurism.
Psithurism(plural not attested)
(obsolete) The sound of rustling leaves.
Origin - An adaptation of the Ancient Greek ψιθύρισµα (psithurisma) or ψιθυρισµός (psithurismos), from ψιθυρίζω (psithurizō, “I whisper”), from ψίθυρος (psithuros, “whispering”, “slanderous”).