I have a Toro Timecutter Z4200 which is about 13 years old. When I shut it off today there was a loud hum from the rear of the machine. I tapped the starter and nothing happen. I then tapped the solenoid and the noise stopped. I started it 3 or 4 times after and it worked fine. I replaced the starter and solenoid a few years ago. Is the solenoid bad?