dī is the beep/drip sound, as in 嘀嗒 (tick-tock). A separate reading dí appears in 嘀咕 (to mutter). Don't confuse 嘀 with 滴, which means a drop of liquid.
口 on the left is the indexing radical. It marks 嘀 as a sound, putting it alongside 嘟 honk and 嘛 modal particle. Whether a drip, a horn, or muttered speech, 口 signals noise through an opening.
啇 on the right supplies the sound, an exact match. It originally pictured the stem-base of a plant and now mostly serves as a phonetic. Same family: 滴 drop, 摘 to pluck, 敌 enemy.
