"After vaccum my furnace(Heat only, Day and Night furnace with model #396GAW00100-ACAA), the fan stop blow but burner will be on when I adjust the temperature below the setpoint on thermostat. My fan had problem last year. It will not stop even after burner off for long time. I had to tap side of the furnace to stop it. It happens 3 or 4 times last year. Is there a relay to control the fan and where it is? how it look like? I have electrical background to fix but need help. Please."

There is a "Heat Sequencer" mounted on the side of the unit that could be at fault.

My best suggestion is to call an HVAC tech and get it fixed properly.

If you start thunking around, you might just induce a static discharge into the unit and fry a control board.