The answer to 'why is my heat exchanger making loud noise' is that a noisy heat exchanger is often a sign of a problem that needs professional attention. Common causes include cracks, corrosion, or a buildup of debris and sediment. This can lead to the heat exchanger cracking or malfunctioning, which can be a serious safety issue.
If you hear loud banging, popping, or other unusual noises coming from your heat exchanger, it's important to have a qualified HVAC technician inspect it as soon as possible. Attempting to repair or service the heat exchanger yourself can be dangerous and should be avoided.