On arrival I spoke with the homeowner, who informed me she was hearing a humming noise from the return air grille. I did find a piece of insulation was loose by the return air plenum. I removed the piece of insulation and cycled power at the thermostat. Customer was not able to hear any noise anymore. Performed a/c tune-up on a Carrier straight cool system. Checked the electrical, mechanical and performance systems of the unit. Cleared, cleaned, flushed and treated the drain line. Used duct strap to secure one of the supply ducts. Secured outdoor compressor wiring and washed the outdoor coil. No major issues were found. The system is operational.