You're used to your furnace running as quietly as it's designed to be. When you hear unexpected noises coming from your furnace, it's naturally a cause for alarm. Loud and unusual noises can be caused by any number of issues. Some of these issues are relatively minor and easily solvable, while others signal the need for a licensed HVAC expert to step in.
The following gives you an idea of what sounds to keep your ears tuned for.