Modern furnaces should always be relatively quiet while they're turned on, so a constant rattling noise coming from your furnace is a sign that something is wrong. A rattling noise is typically an annoyance rather than a cause for extreme concern. However, it's important to find out the source of the noise — if it's coming from the heat exchanger, you'll need to fix it as soon as possible in order to prevent harmful gases from entering your home. To find out what's causing your furnace to rattle and to learn how to repair it, read on.

1. Obstructed or Loose Blower Fan

When your furnace is making a rattling noise, the most common culprit is the blower fan. Get a small flashlight and look into the blower fan assembly inside your furnace. There may be a small screw or bolt that has fallen inside, which will rattle as it's pushed along by the air. Removing it will solve your problem.

If you do find a loose bolt or screw, carefully inspect the inside of your furnace around the blower fan assembly to find out where it came from. Screw it back in tightly.

In older furnaces, the blower fan's bearing can wear out. This causes the fan to become wobbly and unbalanced, which will result in the fan making a constant rattling noise as it spins. You'll need to disassemble the fan and replace the bearing inside in order to quiet it.

2. Cracked Fan Belt

The blower fan is connected to a motor by a fan belt. This belt is typically made of synthetic rubber, and it will break down over time. If it becomes loose, it'll make a squealing noise whenever the blower fan is running. When the belt is cracked, it'll make a rattling noise. In order to solve the problem, you'll need to replace the fan belt — you can do this easily by removing the old one, placing a new one on the blower motor and then tightening it until there's no slack on the belt.

3. Loose Blower Motor

The blower motor is attached to the blower assembly using bolts. If these bolts become loose, vibrations from the motor will make a loud rattling sound as it shakes the fan assembly. You can quickly fix this problem by tightening the bolts with a wrench.

4. Expanding Air Ducts

The rattling sound coming from your furnace may be due to your air ducts instead of the furnace itself. Air ducts are made of metal, so they expand when hot air blows through them and causes the metal to heat up. This can cause the ducts to pull at their supports, which can cause a rattling noise to come from within your home's walls. In order to fix this problem, you'll need to have your air ducts inspected and further reinforced by a heating contractor.

5. Cracked Heat Exchanger

Rarely, the rattling noise may be caused by a crack in your furnace's heat exchanger. If your furnace makes a rattling sound when the fan isn't running, the heat exchanger is most likely the problem. When you have a cracked heat exchanger, you need to have it replaced by a heating contractor as soon as possible — carbon monoxide and other harmful byproducts of combustion can leak out of the crack and into the air inside your home, which is dangerous.

If you can't manage to find the source of the rattling noise or don't feel comfortable fixing your furnace on your own, call a furnace repair contractor in your area. They'll listen to your furnace as it starts up in order to find the source of the rattle, and then they'll make the repairs that are necessary to ensure quiet operation.