Finger tapping

I have a 21 months old boy, who would tap his fingers when he hears loud 'scary' noises like when people are grass-cutting or something similar. He cries for abit, and when i hug him to tell him its okay, he would tap his fingers, softly, not the aggressive kind. Other occasions, when he is concentrating on his cartoon shows, he would do the same. Is there any cause for this, or should i be alarmed? Please advise. TIA. #advicepls #worryingmom #pleasehelp