godan verb (-su)
start to roar; begin to groan; let out a loud noise
Describes the onset of a deep, resonant sound such as an engine roar, a person groaning, or a machine humming loudly.
エンジンが急に唸り出した。
The engine suddenly started to roar.
彼は痛みで唸り出した。
He began to groan in pain.
Standard kanji spelling.
Variant with だす in kana.
Mixed kana-kanji spelling.
唸る is the base verb meaning 'to groan, roar, hum'; 唸り出す adds the nuance of starting to do so.
Compound of 唸り (continuative form of 唸る 'to groan, roar') + 出す (auxiliary verb meaning 'to start doing').