Most people put off furnace replacement until the old unit breaks down beyond repair. This can be a major mistake, though. Instead, consider taking a proactive approach and replacing your furnace sooner rather than later if any of the following issues apply to your heating unit. 1. It's Noisy It can be a nuisance dealing with a noisy furnace. If it is constantly banging, squealing, or rattling, there is likely something wrong with it.