Car won't start

I have a 1988 Camry, automatic transmission and I have to tap the starter to get the car to start. But this is my third starter in six months. I took the starter in and they tested it and said the starter was fine. I have also changed out the key and key ignition switch. When I turn the key there is a humming sound. No clicking. As of today the car won't start at all even when I tap the starter. I just don't know what else to fix or replace now. Can you help?