Funny story for this evening: my dad flew to Missouri to help his elderly aunt and uncle. While out of town, he called my mom morning and evening to catch up. The morning calls always had a horrible hum. The evening calls were fine. My mom asked me to help diagnose the problem. I came by in the morning when the hum was present. After checking a few spots, I unplugged a 25 ft extension cord that went from the front door, down the wall, behind a chair and connected to a phone in the living room. The hum stopped. I traced the wiring behind the chair to find an old 4-pin phone plug and jack. It was discolored. On closer inspection, the discoloration was cat urine. The cat had been peeing on the connector every morning coupling the princess telephone power on the yellow/black pair into the tip/ring green/red pair. By afternoon, the pee had dried and the coupling wasn't apparent. I "retired" the extension cord to the trash can and replaced the plug on the phone. End of problem.
Oh my, that is a funny story about the hum and the cat peeing on the connector in the mornings. One of my sisters worked for Rochester Telephone in Rochester, NY. We always had the new phones when they came out. It was part of the benefit of her working for the phone company. She retired in 1992 at the age of 50, having worked there for 32 years. She got her monthly phone service for free until the day she died in 2011.