Though your furnace will make some normal noises when it first comes on or shuts off, listen for any strange, loud noises, as these could indicate a need for furnace replacement. Often, an old or damaged furnace fan is behind the racket. Rattles, bangs, clunks, and squeals could all indicate that furnace replacement is in your near future. Your local furnace expert will be able to diagnose if these noises call for furnace repair or full-on replacement.