It's easy to grow accustomed to a noisy furnace over time. At first, those bangs and clangs bother you, but after a few weeks, they just become part of the background noise in your home, and you basically tune them out. As natural as this process may be, it's not a smart one to follow. Furnaces only become noisy when something is wrong. There is a good chance your furnace needs one of the following repairs or services.
Ball Bearing Replacement
The motor that drives the fan portion of your furnace has a few ball bearings that allow the parts to slide over each other without much friction. These ball bearings can become worn down over time, and once they do, friction in the unit increases. This leads to a lot of scraping noises and clunking noises. Luckily, ball bearings only cost a few dollars, and it will only take your furnace repair company a couple of hours to put a new one in place, so you can have a quiet furnace again.
Motor Belt Replacement
There is also a belt that drives the furnace motor, much like the timing belt in your car's motor. If this belt starts fraying, which is pretty common once the belt ages, it will begin to make a screeching or squealing noise. The furnace repair technician can pretty easily undo a few screws, pop the old belt off, and put a new belt in its place.
Main Duct Tightening
Another possibility is that the main duct that carries all of the warmed air out of the furnace blower is loose. This is probably the easiest issue to correct, as your furnace technician will just need to tighten a few screws or bolts. That will keep the duct from bouncing around every time air blows through it.
Burner and Ignition Cleaning
If the banging noise is most noticeable when your furnace first switches on, then what you're hearing may actually be a buildup of natural gas igniting all at once. This can occur when the burner or ignitor are dirty. Your furnace repair company can easily take apart the burner and clean it, which should solve the problem.
If your furnace is noisy, don't just ignore the noise it makes. Take action. Call a local furnace repair service, and have them look into the problems above as possibilities. They'll find the source of the noise and fix it.