I inspected the furnace today and found that the noise was coming from the return drop. The return drop has gotten a week overtime and with it being undersized for the size of the system, the furnace is pulling hard to get air through the return duct, which puts act extra strain on the ductwork. I performed a thorough inspection of the system. The furnace is running safe as of today. I did find multiple spots on the heat exchanger where the metal is starting to bevel or chip away. I showed customer the spots and they would like to keep this system running for now. Customer would like to revisit this conversation in about a month.