I have a 1/2 hp ridgid sewage pump that hums intermittantly. It only does it for awhile after the tank pumps out. It will hum for about 10 seconds than shut off for about 3 seconds, then start again. This happens regardless of the float being plugged in or if the pump is plugged in directly. If the pump is not used for awhile, 20 minutes or so, the humming will eventually stop. Any answers?