Is there an idiomatic meaning for 口の中?

Question

I read this phrase in a book:

・・・・[彼]は口の中でうなった。

I'm guessing the うなった is 唸った, or groan, but I'm curious as to if 口の中 has a meaning or implication other than the obvious, inside his mouth. How does one groan if not inside one's mouth? My best guess is that it's something along the lines of "under his breath"...but that still doesn't seem right to me.

For context, the 彼 is (something like) a high priest who's speaking with the emperor (which is what makes me think he is trying to suppress a groan out of respect).

Answer 1

口の中で is probably the Japanese idiomatic equivalent of "under one's breath." Literally, neither idiom makes sense, but both sort of convey the same concept.

彼は口の中で唸った。// "He groaned under his breath."

Answer 2

ssb hinted at the answer, but basically 中 implies a closed-off space. So in your example 口の中 is used metaphorically to mean that the person is groaning (making a sound) with there mouth closed to imply someone is thinking deeply about something, expressing there displeasure about something, etc.