The customer reported that their furnace was producing a loud noise during operation, which would stop once the blower activated. I began by replacing the nozzle and verifying the electrode settings, which were correct. After re-firing the furnace, the noise persisted. I then removed the blower assembly and drilled two inspection holes in the supply plenum to perform a camera inspection. The heat exchanger was found to be heavily rusted; however, no visible cracks or holes were detected. I pulled the blower out of the back of the furnace and looked at the heat changer as much as I could visibly see and did not see any holes or cracks. I tested multiple nozzle sizes and adjusted the pump pressure, but the noise did not improve. I ultimately returned to the original 0.75 x 80A nozzle and completed a full combustion analysis. I also performed multiple heat-exchanger tests, monitoring combustion values with and without the blower running. Combustion readings remained stable and did not shi
