When your furnace and the accompanying duct work are behaving as they should, the only noise you should hear is the sound or air being forced through your vents. If your furnace makes a loud banging noise when it kicks on, then something is amiss. There are two possible causes for this issue -- each of which is rather easy for your HVAC technician to fix.   

Cause #1: Delayed gas ignition.

If too much gas builds up in the chamber of the furnace before it ignites, this may result in a "boom" noise when the gas does finally light. This noise is actually the sound caused by a mini-explosion in your furnace. If the banging you're hearing consists of a single "bang," this is likely the cause.

There are several reasons why too much gas may be building up in the chamber. First, your burners may be dirty or worn out, making them unable to light until more gas builds up. Second, your pilot light may be old and weakening. A professional can replace your pilot light or burner, and this should fix the issue. Since these repairs must be completed perfectly in order to prevent accidents such as explosions and gas leaks, they should not be attempted by the average homeowner.

Cause #2: Expanding and contracting ducts.

When metal is exposed to heat, it expands. When it cools, it contracts. If your ducts are not properly sized in relation to one another, they may make a banging noise when one bumps into another during this contraction and expansion. Often, banging caused by this issue consists of multiple, smaller bangs in a single heating cycle.

You may also sometimes hear this type of banging if your ducts are dirty -- the dirt buildup in one duct can cause it to absorb more heat, which causes it to expand at a different rate than the other ducts, leading to a banging sound. Having your ducts cleaned and looked over by a professional is the best solution. He or she can replace any ducts that are improperly sized, and perhaps even install a layer of insulation over certain ducts to prevent them from banging.

A loud, banging heating system is not something you should have to live with. If your system is making noises that keep you up at night, don't delay in contacting a local HVAC technician (such as one from Super Cool Heating & Air) and having something done about the problem.