Odd noises from your home's furnace should never be ignored. Often the problem is repairable, but you can minimize damage by making an immediate service call to a heating contractor.
1. Startup Boom
A boom or thud upon startup is generally a problem with a gas furnace. The cause is residue buildup in the ignition or burner plate. When the ignition clicks on, this residue ignites and causes the noise. If you don't have a gas furnace, then the issue could be a loose duct that is moving into place loudly only when the air begins to blow through it.
2. Initial Rumbling
Rumbling noises that start when the furnace powers on and continue for a few seconds to a couple of minutes are a sign of grime and dust on the heating elements. The problem primarily affects electric furnaces, but it can also occur with a gas furnace if the burners are dirty. The rumbling is a result of system strain, and it can be repaired with a full maintenance check and cleaning.
3. Persistent Grinding
Grinding noises indicate a more severe service problem. This type of noise usually originates from the air handler, the mechanized part of the furnace that moves air through and blows it into your home once heated. Grinding indicates that something in the air handler's moving parts is beginning to fail. The fix could be as easy as an adjustment and lubrication, or your repair service may need to replace all or part of the air handler's components.
4. Repeated Clicks
A furnace typically clicks when it first comes on, so it is a normal but infrequent noise. If your furnace clicks repeatedly, then something is wrong with the temperature sensor and causing it to short cycle. The cause could be as simple as a broken thermostat, or it may involve issues with the heating elements not heating the air to the desired temperature. A full inspection is needed to pinpoint the exact issue so that it can be repaired.
5. Annoying Squeal
All furnaces have some sort of motor to run the air handler and circulate air. Lack of lubrication, broken components, or dirt in the system can cause squeals to come from the motor. Some motors also have a fan belt, which can squeal as it begins to give out. The repair depends on which part is specifically making the noise.
Contact a furnace repair service if your furnace is making any type of odd noise.