Performed the level 2 furnace cleaning. The loud noise that the customer complained about is the venter motor. The motor bearings are worn and expected to fail. Upon further inspection, I found water damage in the blower compartment, possibly from the secondary heat exchanger. The primary heat exchanger is rusting and chipping. CO in the exhaust was larger than normal at 88 ppm. Average life of system is 12-15 years. Unit is currently 18 years old. Due to age, condition, and cost to repair, second opinion is recommended. Failure is expected. Even though the furnace is safe to run now, we cannot guarantee how long it will last after we leave today.