Underbreath

Muttering curses underbreath, he jabs the beast sharply with his training stick ; the elephant is hairy and it, like the clown, is ignoble.

Source: wiktionary

The name, Night Stalker, seemed to find favor with Bo who repeated it underbreath several times.

Source: wiktionary

Here, on the contrary, we murmur underbreath.

Source: wiktionary

Aleck gave a low, underbreath whistle.

Source: wiktionary