咕 is almost always reduplicated as 咕咕 (gūgū) for sounds: 咕咕叫 (stomach rumbling, pigeon cooing). It also appears in 咕噜 (gūlu, gurgling/rolling sound) and 咕哝 (gūnong, to mumble/mutter).
Mouth radical on the left, the indexing component. In 咕 it does double duty: it signals an onomatopoeia (sound from the mouth) and marks the family of bubbling/animal-noise chars — 咚 thud, 咯 cluck, 嘟 mumble. If left-side 口 sits next to a sound-like phonetic, expect a noise word.
Right side 古 gǔ supplies the sound with a small tone shift gǔ → gū (often in onomatopoeia tone marks blur). The same phonetic surfaces in 估 estimate, 姑 aunt, 故 reason, 苦 bitter — all sharing the gu syllable. Meaning is purely sound here, no 'ancient' flavour.
