The answer to 'heat exchanger making loud noise' is that a noisy heat exchanger is often a sign of a serious problem that requires professional repair. The most common causes are a cracked heat exchanger, worn bearings in the blower motor, or a buildup of debris and dirt. A loud, rumbling, or banging noise coming from the heat exchanger indicates that it may be failing and needs to be inspected and repaired immediately.
While some minor issues like a loose part may be fixable with DIY maintenance, a noisy heat exchanger is generally not something a homeowner should attempt to repair on their own. Trying to continue running a furnace with a damaged heat exchanger can be extremely dangerous, as it can lead to carbon monoxide leaks. For your safety, it's best to call an HVAC technician as soon as you notice any unusual noises coming from your furnace's heat exchanger.