Furnace blowers are comprised of fans and the motors that power them and blow heated air into the duct system of your home. When they fail to function properly, they can cause intense vibrations or rattling that travels through the metal ductwork and causes annoying noises in different parts of the home.
Bent or Unbalanced Blower Wheel
The blower wheel is the main part of the fan on a furnace blower unit, usually found in the form of a large metal wheel with small fan blades attached to the inside of it. Blower wheels must spin evenly to avoid making noises or high-speed vibrations during operation. Dirt, dust and soot build-up can unbalance the blower wheel, says R.S. Andrews Services, so regular cleaning must be performed. Some furnace issues can bend or otherwise damage the fan blades of the blower wheel, such as the high force combustion known as a puff back that oil furnaces sometimes cause, causing serious blower vibration and requiring a replacement wheel.
Both the blower wheel and motor of the furnace blower contain bearings that can become damaged by friction as the oil or grease on them wears away. When the bearings in a blower assembly wear out or stop altogether, the unit creates grinding or whining noises and can shake and vibrate greatly, according to Inspectapedia. Some bearings require regular applications of oil or grease and will not last long without this regular maintenance. Other types of bearings are oiled once during installation and should last for many years without replacement.
Furnace blower motors are connected to and move the fan in the unit with a belt, which is usually made of rubber or a similar material and prone to damage after years of constant use. Loud squeals or shrieks, accompanied by erratic or constant vibration, signal a damaged or tearing blower belt. Homeowners can replace their own blower belts, says HomeTips, as long as the unit is completely shut off before attempting the repair. You can also inspect the belts yourself easily. Check the tension of it against the tension recommended in your furnace's user manual, and look for cracks, tears or signs of damage in the belt material.
Blowers produce a very small amount of vibration at all times during normal operation, and this constant low shaking can loosen the screws and nuts attaching the blower to the furnace, according to the Ductwork Installation website. If other areas of the blower are running well and in fine condition, tightening all of the fasteners on the unit may stop excess vibration. Some blower frames or cages also feature small pieces of rubber or foam to absorb vibration. If these have been lost, replace them.