When plants are water-stressed, water in their xylem (their water-conducting veins) can be under so much tension that the air dissolved in the water expands explosively to fill the encasing xylem unit. This is known as cavitation. And with the right equipment, or the right trees and conditions, you can hear it.
BoingBoing has a delightful recoding of a one tree cavitating. Just ignore the explanation – it’s wrong.
Bioacoustician Bernie Krause, source of the above recording, is also seen here describing how he came upon the sounds (but again, ignore the explanation):
PS. This has nothing to do with bluegrass, which is pretty much devoid of percussion instruments.