A broken furnace can catch fire. If your furnace's gas pressure is too high explosive ignition can occur. If there is a crack in your furnace's heat exchanger explosive ignition can occur. If you find any abnormal condition, these include high pitched noise, grinding, rumbling, popping, squealing, or banging noises you should repair the cause of the condition without hesitation. Shut down and find the problem. The condition could be the result of small issues like loose components within the system, more serious issues or an easy repair if ignored causing a costly repair or catastrophic failure of the system. Either way, you should attend to the repair promptly or call a professional to look into the issue.