The term 'fluttering sound' can be described by two words: TRILL and WHIRR. A TRILL is a rapid alternation between two musical notes, usually a half step or whole step apart. It is often used to create a vibrato effect. A WHIRR is a continuous, high-pitched sound that is produced by a rapidly rotating object, such as a fan or motor. Both of these sounds are associated with fluttering, and together they create a unique soundscape.