嗒 is almost always used in reduplicated form 嗒嗒 (dādā) to represent repetitive tapping/clicking sounds. Also appears in 吧嗒 (bādā) for the sound of smacking lips or dripping.
口 depicts an open mouth — the conventional flag for onomatopoeia. As radical it marks 嗒 as a noise-rendering character, joining 哒嘀嗡啪, all written sound-effects produced or imitated by the mouth.
荅 supplies the sound (dá shifted to dā). 荅 is an old variant of 答 with the original grass-radical-on-bean meaning. Here it serves purely as phonetic — the mouth header signals the character is being used for its tap-tap sound.
