A furnace can’t just tap somebody on the shoulder and say it’s not feeling well. It can, however, make weird noises when problems come up—noises that often provide valuable clues as to what’s going on. See, the blower wheel, the ducts, and even the burner produce odd or loud sounds when they need to be repaired, sounds homeowners are likely to notice. 

A Thump in the Night
One of the most common weird noises furnaces make is a thump, which can even sometimes sound like a vibration, or a crooked washing machine when all the wet clothes shuffle to one side of the drum. Just like in the washer, this probably means the blower wheel in the HVAC system is unbalanced. Or, a bit more severe, the motor has gotten off track. Whichever part is the culprit behind that thumping, the problem is misalignment.
This is a serious issue, although it probably doesn’t warrant a full furnace replacement, at least not in most cases. You should still, however, get it checked as soon as possible because out-of-balance blower wheels and motors that aren’t corrected can create bigger problems down the line, leading to a furnace suddenly quitting.
Another potential cause for the thumping is a bad indoor fan or capacitor, especially if the furnace is also refusing to blow hot air. The capacitor is the component of your furnace that brings it electricity. This means that under no circumstances should you attempt to service it yourself, as a repair will call for a professional with a multimeter and other tools. Always play it safe when there’s voltage involved and bring in an HVAC expert.

The Scrapes of Wrath
Of all the sounds an out-of-whack furnace can make, the harshest one is a loud, metallic scraping. Imagine the universally reviled noise of fingernails on a chalkboard. This one is a bit like that, except it involves sharp metals. While the thumping I mentioned can be subtle and sometimes easy to miss, this is easily noticeable. It may also indicate an issue with the blower, specifically that it has started to hit the housing as it rotates. This is worse than the misalignment we mentioned above because it means it’s become so loose that it’s now making prolonged contact with other pieces.
Homeowners always act surprised when I say this, but it’s actually a good thing this sound is so severe. If you don’t shut your furnace down when you hear it, your blower wheel can permanently damage the inside of your HVAC unit, even denting the structural housing, which often forces a furnace to need to be replaced before its time. I wouldn’t run the furnace on even the coldest of nights after I heard this sound. I’d shut it down and call a professional technician.

It’s a Pop, It’s a Bang… No, It’s a Problem with the Ductwork
If you hear a loud pop or bang each time your furnace starts or shuts down, you probably have ductwork that’s expanding and contracting. When the furnace starts, it pulls in air through the ducts, which requires negative pressure. That drop in pressure causes your sheet metal ducts to suck in, and they give off that loud noise. It then happens again when the furnace stops and the pressure returns to normal, letting the ducts expand and creating another loud bang or pop.
This means your ducts may be too flimsy to handle the necessary pressure changes for your furnace. They could also be undersized or poorly braced, both of which can cause them to shift. Whatever the cause, this problem isn’t as serious as the previous two, as it doesn’t involve the innards of the HVAC system, but it’s worth correcting. Bad ducts can lead to lower energy efficiency or even leaks that drop dust into your home. You certainly don’t want your electricity bill to go up—or the air quality in your home to plummet.

SOURCE: bellbroshvac.com

A furnace can’t just tap somebody on the shoulder and say it’s not feeling well. It can, however, make weird noises when problems come up—noises that often provide valuable clues as to what’s going on. See, the blower wheel, the ducts, and even the burner produce odd or loud sounds when they need to be repaired, sounds homeowners are likely to notice. 

A Thump in the Night
One of the most common weird noises furnaces make is a thump, which can even sometimes sound like a vibration, or a crooked washing machine when all the wet clothes shuffle to one side of the drum. Just like in the washer, this probably means the blower wheel in the HVAC system is unbalanced. Or, a bit more severe, the motor has gotten off track. Whichever part is the culprit behind that thumping, the problem is misalignment.
This is a serious issue, although it probably doesn’t warrant a full furnace replacement, at least not in most cases. You should still, however, get it checked as soon as possible because out-of-balance blower wheels and motors that aren’t corrected can create bigger problems down the line, leading to a furnace suddenly quitting.
Another potential cause for the thumping is a bad indoor fan or capacitor, especially if the furnace is also refusing to blow hot air. The capacitor is the component of your furnace that brings it electricity. This means that under no circumstances should you attempt to service it yourself, as a repair will call for a professional with a multimeter and other tools. Always play it safe when there’s voltage involved and bring in an HVAC expert.

The Scrapes of Wrath
Of all the sounds an out-of-whack furnace can make, the harshest one is a loud, metallic scraping. Imagine the universally reviled noise of fingernails on a chalkboard. This one is a bit like that, except it involves sharp metals. While the thumping I mentioned can be subtle and sometimes easy to miss, this is easily noticeable. It may also indicate an issue with the blower, specifically that it has started to hit the housing as it rotates. This is worse than the misalignment we mentioned above because it means it’s become so loose that it’s now making prolonged contact with other pieces.
Homeowners always act surprised when I say this, but it’s actually a good thing this sound is so severe. If you don’t shut your furnace down when you hear it, your blower wheel can permanently damage the inside of your HVAC unit, even denting the structural housing, which often forces a furnace to need to be replaced before its time. I wouldn’t run the furnace on even the coldest of nights after I heard this sound. I’d shut it down and call a professional technician.

It’s a Pop, It’s a Bang… No, It’s a Problem with the Ductwork
If you hear a loud pop or bang each time your furnace starts or shuts down, you probably have ductwork that’s expanding and contracting. When the furnace starts, it pulls in air through the ducts, which requires negative pressure. That drop in pressure causes your sheet metal ducts to suck in, and they give off that loud noise. It then happens again when the furnace stops and the pressure returns to normal, letting the ducts expand and creating another loud bang or pop.
This means your ducts may be too flimsy to handle the necessary pressure changes for your furnace. They could also be undersized or poorly braced, both of which can cause them to shift. Whatever the cause, this problem isn’t as serious as the previous two, as it doesn’t involve the innards of the HVAC system, but it’s worth correcting. Bad ducts can lead to lower energy efficiency or even leaks that drop dust into your home. You certainly don’t want your electricity bill to go up—or the air quality in your home to plummet.

SOURCE: bellbroshvac.com