Nobody enjoys going into a dark basement when there are alarming sounds coming from the furnace. Here are some likely sounds that might be making the noise. If you hear a loud ‘boom’, it’s usually caused by a delayed ignition. An Heating plus Air Conditioning business should be called instantaneously since the cause could be an explosion from gas building up in the furnace chamber. The same sound occurs when too much gas builds up when lighting a gas grill. There are several explanations for why ignition is delayed in a gas furnace. One is that there is a high air-to-gas ratio, making the ignitor difficult to light. Another cause might be insufficient amounts of gas due to low gas pressure. Finally, the burners could be jammed with debris, worn down, or otherwise compromised. Another sound coming from the furnace is more like a house-shaking rumble as the furnace completes one of its heat cycles. This cause is really due to the fact that oil continues to burn in the combustion chamber after the burners shut off. Again, call a certified Heating plus Air Conditioning repairman instantaneously. This problem can lead to carbon monoxide gas being released into the home, which is deadly. If the furnace makes a loud rattle when it clicks on, carbon monoxide could also be the culprit. The furnace may have a cracked heat exchanger, which is the largest cause of carbon monoxide leaks. Next are squealing or grinding noises, the cause which more than likely is a loose blower belt. Or, the motor bearings could be failing and need to be replaced or oiled. A clicking noise is normal when a furnace first turns on, which is from the ignitor. Finally, a buzzing noise is really the result of something that’s loose in the furnace transformer.