A faulty heat exchanger will frequently emit an unpleasant and pungent stench that smells like formaldehyde. Another sign of a failing heat exchanger is audible noises. When the thermostat switches on the heat, you'll probably hear a rattling noise if the heat exchanger is cracked. So because the heat exchanger is formed of metal, cracks will expand and contract as it heats up, resulting in creaking, cracking, and pounding noises.