Is there an idiomatic meaning for 口の中?


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).




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."

