Said another way, an instantaneous change in gain causes a step change in the audio signal. A step change isn't physically possible, but is equivalent to the su

Said another way, an instantaneous change in gain causes a step change in the audio signal. A step change isn't physically possible, but is equivalent to the summation of harmonic frequencies up to infinity. Your speakers try their best, and end up playing all the harmonic frequencies within their range of 20Hz-20Khz or so. This happens to produce a similar sound as a percussion instrument.