groan, roar
唸 centers on a deep, rumbling vocal sound: a groan of pain or a roar of anger. Both meanings share the idea of a low, forceful utterance coming from the throat.
唸 combines 口 (mouth) with 念, which likely contributes the sound. The character expresses a vocal sound made with the mouth, fitting the meanings groan and roar.
A mouth 口 making a deep, rumbling sound like a groan or roar, with 念 suggesting the sound is held in the throat.
For テン, imagine a tense groan: ten -> テン, and the mouth 口 lets out a low, tense roar.