Is the humming coming from your rigger or the cable? I can't really think of anything that would make a manual rigger hum. Now the cable is a different story. The cable hums all the time. Depending on how fast you're trolling.

I thing the fact that the fish "disappeared" off your screen may have been a coincidence. You may have trolled thru & out of a school of fish. But you didn't leave the rigger down long enough to test the theory.

Give it another try. You should be fine.