1995 Toyota Corolla What is the problem with my distributor?

1995 Toyota Corolla mileage: 65,500. What is the problem with my distributor? I have heard repeating noise from it when the engine running. I opened it and saw the rotor edge was damaged, so I replaced a new rotor that I bought from a local Toyota dealer. But the noise still won't go away. I also check ignition coil's primary resistance, which was 0.7 ohm [within specification?]. The secondary resistance was 12.51 K ohms. Additional information: 1. In Aug 2000, a local Toyota dealer installed new timing belts and new starter for my car. 2. Only the silicon portion that was connected to spark plug in engine # 1 turned to dark orange color. It was black color before the noise symptom. 3. My car had a new distributor at 32,500 miles.