mumble, mutter, murmur, rumble
咕 centers on low, indistinct vocal sounds: mumbling, muttering, murmuring, or a low rumble. All meanings share the idea of quiet, repetitive, or unclear speech-like noise.
咕 combines 口 (mouth) with 古, which likely contributes the sound コ. The character is used to represent low, rumbling vocal sounds.
A mouth 口 making an old 古 sound: imagine an old person mumbling and muttering quietly, producing a low rumble.
For コ, picture a cooing pigeon: the soft 'coo' sound matches the mumbling, murmuring idea of 咕. Coo -> コ.