Tech found the popping noise was from the back pressure on the supply and return plenums. Tech tested external static pressure and found it at .64 which is slightly high. Pressure drop across coil was .08. Tech verified blower wheel not dirty, coil is clean and filters not clogged. The only repair would be replacing the ductwork. Tech found the furnace is 20 years old and the coil and condenser are both 1989 units. Tech found coil in poor condition, primary and secondary pans both rusted out, drain not insulated, compressor failed megohm test and compressor terminal wire partially melted. Tech recommends replacing unit before ductwork is replaced. Unit is operational at this time. Customer declined replacement appointment. Wants to speak with office