That's right — the abnormal noise was the cooling fan's final death rattle.
Literal
That's-right, abnormal-sound [topic-は] air-cooling-fan [possessive-の] last death-agony was.
断末魔 (death agony/death throes) is a dramatic Buddhist-origin term used here with dark humor about a failing computer fan. The metaphor of a mechanical part dying in its 断末魔 is vivid and playful. ~のです adds a revelatory tone: 'so that's what it was.'