1. The blades has accumulative dust, or are corroded and eroded by  wind, thus causing the induced draft fan to lose  dynamic balance.
2. Damage of bearing.
3. The fan shaft dynamic balance is not good.
4. The flue has resonance. Flue resonance is the problem that the new furnaces commonly have,  other problems will happen after a period of running. The new fan unqualified phenomenon shall not be ruled out.