Rattling noises coming from the furnace may be caused by dusty burners or air filters, a lack of insulation, a damaged furnace belt, ducts warping or expanding, loose blower motor set-screws, or a faulty draft inducer motor. Most of these issues have easy fixes that you can perform by yourself.

The Ignition Is Being Lit
If you are hearing a brief clicking or rattling noise before the furnace turns on, this is probably just the sound of the furnace’s ignition being lit.  It’s nothing to worry about as long as it stops before the furnace starts.
However, if you hear a repeated clicking sound and your furnace will not turn on, the igniter is likely failing to start the pilot light.  This means that the ignition system itself is malfunctioning, i.e. failing to energize.
The ignition system has a lot of moving parts, so fixing it is a problem best left to a professional HVAC repair service.

Dusty Burners or Air Filter
If your furnace has accumulated an excessive amount of dust in the burner or air filter, that could be causing the rattling sounds. Dust and debris in these moving parts can lead to a surprising amount of noise, especially if it’s been more than a year since your furnace has been cleaned.
The first step is to check your blower and burner cavity for dust, making sure to clean off any dust on the blower fan and in the back corners of the cavity. You can remove the dust with a handheld or hose vacuum, or use a drain line for especially hard-to-reach places.  Be sure to clear dust from the blower motor itself as well.
Once you’ve cleaned the blower motor, you should check the air filter. If it’s noticeably dusty, it’s time to replace it, as air filters are designed to collect dust and need to be replaced frequently.  This can easily be done yourself—just make sure to purchase the right size for your furnace.  I recommend using Filtrete Clean Living Basic Dust Filters (available on Amazon.com). They’ll catch more dust than alternative filter materials like fiberglass, and one of the 20 available sizes is sure to fit your furnace.

Stiff or Defective Furnace Belt
You may notice that your furnace belt is cracked or in disrepair when dusting the body of your furnace.  If so, you should go ahead and replace it, as a stiff or defective belt may be contributing to the rattling noise.
Belts can be purchased online or at your local hardware retailer.  I recommend getting the D&D PowerDrive Rubber Belt (available on Amazon.com) as it’s a cost-effective, durable, and reliable option.
Once you’ve got your new belt, you can follow these steps to replace the old one:
Carefully loosen the bolts of the pulley.  No need to remove them entirely, just enough that you are able to take the old belt off.
Place the new belt on the pulley, fitting the motor side on first.
Test the tension of the belt with a screwdriver or your finger.  The belt should give only 1/2 inch when secure.
Turn the furnace on and listen for any strange noises.
If the new belt continues to make noise, adjust the tension of the belt until the noise dissipates.  

Ducts Warping or Expanding
If what you are hearing is an occasional banging/rattling noise as your furnace runs, it’s possible that your ducts are simply warped after long-term use.  This doesn’t affect furnace operation, but it can be irritating. 
Believe it or not, a good duct cleaning can actually cause more of these popping noises—the dust can act as a kind of internal insulation, and when it’s gone the ducts have less weighing them down.  Again, this is nothing to worry about.
However, if the noise is extremely frequent or loud, you may want to insulate your ducts, as this may muffle the sounds. This video can help you with DIY insulation.  (Note that duct tape should not be used on HVAC ducts!)

SOURCE: temperaturemaster.com

Rattling noises coming from the furnace may be caused by dusty burners or air filters, a lack of insulation, a damaged furnace belt, ducts warping or expanding, loose blower motor set-screws, or a faulty draft inducer motor. Most of these issues have easy fixes that you can perform by yourself.

The Ignition Is Being Lit
If you are hearing a brief clicking or rattling noise before the furnace turns on, this is probably just the sound of the furnace’s ignition being lit.  It’s nothing to worry about as long as it stops before the furnace starts.
However, if you hear a repeated clicking sound and your furnace will not turn on, the igniter is likely failing to start the pilot light.  This means that the ignition system itself is malfunctioning, i.e. failing to energize.
The ignition system has a lot of moving parts, so fixing it is a problem best left to a professional HVAC repair service.

Dusty Burners or Air Filter
If your furnace has accumulated an excessive amount of dust in the burner or air filter, that could be causing the rattling sounds. Dust and debris in these moving parts can lead to a surprising amount of noise, especially if it’s been more than a year since your furnace has been cleaned.
The first step is to check your blower and burner cavity for dust, making sure to clean off any dust on the blower fan and in the back corners of the cavity. You can remove the dust with a handheld or hose vacuum, or use a drain line for especially hard-to-reach places.  Be sure to clear dust from the blower motor itself as well.
Once you’ve cleaned the blower motor, you should check the air filter. If it’s noticeably dusty, it’s time to replace it, as air filters are designed to collect dust and need to be replaced frequently.  This can easily be done yourself—just make sure to purchase the right size for your furnace.  I recommend using Filtrete Clean Living Basic Dust Filters (available on Amazon.com). They’ll catch more dust than alternative filter materials like fiberglass, and one of the 20 available sizes is sure to fit your furnace.

Stiff or Defective Furnace Belt
You may notice that your furnace belt is cracked or in disrepair when dusting the body of your furnace.  If so, you should go ahead and replace it, as a stiff or defective belt may be contributing to the rattling noise.
Belts can be purchased online or at your local hardware retailer.  I recommend getting the D&D PowerDrive Rubber Belt (available on Amazon.com) as it’s a cost-effective, durable, and reliable option.
Once you’ve got your new belt, you can follow these steps to replace the old one:
Carefully loosen the bolts of the pulley.  No need to remove them entirely, just enough that you are able to take the old belt off.
Place the new belt on the pulley, fitting the motor side on first.
Test the tension of the belt with a screwdriver or your finger.  The belt should give only 1/2 inch when secure.
Turn the furnace on and listen for any strange noises.
If the new belt continues to make noise, adjust the tension of the belt until the noise dissipates.  

Ducts Warping or Expanding
If what you are hearing is an occasional banging/rattling noise as your furnace runs, it’s possible that your ducts are simply warped after long-term use.  This doesn’t affect furnace operation, but it can be irritating. 
Believe it or not, a good duct cleaning can actually cause more of these popping noises—the dust can act as a kind of internal insulation, and when it’s gone the ducts have less weighing them down.  Again, this is nothing to worry about.
However, if the noise is extremely frequent or loud, you may want to insulate your ducts, as this may muffle the sounds. This video can help you with DIY insulation.  (Note that duct tape should not be used on HVAC ducts!)

SOURCE: temperaturemaster.com