manacles
梏 centers on physical restraint, specifically wooden handcuffs or manacles used to bind a person.
梏 combines 木 (wood) with 告, which likely contributes the sound. The character originally referred to wooden manacles.
The left side 木 is wood, and the right side 告 looks like a person with arms outstretched. Picture wooden manacles clamping around those outstretched arms.
For コク, imagine a prisoner in wooden manacles knocking them against a wall: knock -> コク, and the sound echoes as the manacles hit the wood.