Rattling noises from your home's furnace shouldn't be ignored. Although most cases are minor, some can be dangerous to your or to the furnace.
1. Loose Housing
One of the most common culprits of rattling is that something is loose on the housing, most often the access panel. The rattling noise will start up a moment or two after the furnace pops on. Check the access panel to make sure it is fully seated in place. Many furnaces have a safety feature that shuts down the furnace completely if the access panel opens up, so you want to make sure it is completely latched closed. Then, check any screws or hardware on the exterior of the housing to make sure they are properly tightened.
2. Cracked Heat Exchanger
If the rattling begins at the same time the furnace pops on, or even a second or two before you hear the furnace come on, then the problem could be a cracked heat exchanger. The cause of the rattling is the thermal shock causing the cracked metal of the exchanger coils to move about. This isn't something you can ignore, since cracks in the heat exchanger can lead to leaking carbon monoxide. A prompt call for a repair is needed before you run the furnace again.
3. Blower Issues
The main moving part inside the furnace is the blower assembly, so it is often the source of any rattling noises. There are several issues that can cause a blower to rattle. If there is any damage to the fan or if it is out of balance, you may hear rattling when the blower comes on. Damage to the ball bearings inside the assembly may also cause rattling, as can a loose or broken blower mount. A maintenance inspection will be necessary to locate the source of the blower issue so that it can be repaired.
4. Shaking Ductwork
Sometimes the issue with rattling isn't directly from the furnace but instead, it's in the ducts that the furnace feeds into. Duct rattling usually begins a moment or two after the furnace comes on and continues for the entire heating cycle. The cause could be as simple as a loose duct connection into the furnace, or it may be coming from a loose connection in the ducts within the walls. Your repair tech may need to perform a duct inspection to find the source.
Contact a heating repair service if you notice a rattling noise coming from the furnace.