Abraham Cowley, the 17th-century poet, was a country boy at heart. Of the city people, pursuing the elusive and ephemeral "honey of all earthly joy", he says:
They, methinks deserve my pity,
Who, for it, can endure the stings,
The crowd, the buzz and murmurings,
Of this great hive, the city.
Now, it is interesting that Cowley should choose a hive of bees as a metaphor for city noise. When we hear a swarm of bees, the aggregate buzz is pitched to that of the "average" bee, but the group can be heard far beyond the audible range of any one inhabitant of the hive.
Likewise presumably for crowds of people, and in the same phenomenon can be found the key to the noise of the wind as it whistles through trees.
Let us first recall a sound that nowadays we seldom hear, that of a telephone wire humming in the breeze. A wire stretched before the wind like this disturbs the flow of air around it, causing it to become unstable and to break up into a sequence of little eddies that are carried on downstream. These "flutterings" of the air are sometimes fast enough to be heard as a musical note.
The flutter frequency increases with wind speed, thus increasing the pitch of the audible note, and decreases the greater the wire's diameter. Sometimes, if wind is strong enough and the eddy-frequency is close to that of the natural frequency of the wire itself (the rate at which it would oscillate on its own if plucked) the alternating variations in pressure above and below the wire cause it to vibrate.
This vibration is not necessary for sound to be produced, but when present it enhances the volume, and makes the sound more persistent by prolonging it during lulls in the wind, when the hum might otherwise momentarily die away.
In much the same way, as the wind passes around a myriad of twigs and branches in a wood, the resulting eddies produce notes that vary widely in their pitch and volume. Large twigs provide a low rumble, while the tiny needles of a conifer produce a high-pitched hiss.
And, as in the case of bees, the blend of sound we ultimately hear comprises, as it were, the average pitch of all the individual notes, but at a loudness that is the combined volume of the sum all the individual sounds.
Although the note of a single conifer needle may be inaudible even at very close range, the forest as a whole is often heard a mile or more away.