The word "窸窣" is formed as an onomatopoeic compound where both characters are used purely for their phonetic values to mimic a light, rustling sound. Each character incorporates the "穴" radical, but in this context, the radical does not contribute to meaning; instead, the sounds "xī" and "sū" are combined to audibly represent the noise, resulting in a disyllabic word constructed solely through sound imitation.