The answer to 'why is my heat exchanger making loud noise' is that a loud noise from the heat exchanger is often a sign of a problem that requires professional repair. Common causes include a cracked or damaged heat exchanger, debris buildup, or issues with the blower motor.
A noisy heat exchanger can indicate a serious safety hazard, as a cracked heat exchanger can leak dangerous carbon monoxide into your home. If you hear any unusual noises coming from your furnace, it's important to shut it off immediately and call an HVAC technician to inspect and repair the issue.