What's the word I'm thinking of? Today, it's....
PSITHURISM [SIGH-thur-IS-em] (noun)
The sound of rustling leaves or wind in the trees.
"Another day the sweet south is blowing; do you not see how the larch and lime palpitate with pleasure? ... do you not hear the musical psithurism of the feathered foliage?"
--Mortimer Collins, "The Secret of a Long Life" (1875)