Check the solenoid. Yesterday my truck started making this buzzing/grind noise while going down the road. I took it to my mechanic and he replaced the solenoid. It could be what you are experiencing. They get water in them, I guess and it ruins them. There's a TSB on it I guess.
2005 Ford F-150 Lariat Supercrew (dark stone metallic) - Corsa DB Exhaust
1994 Chevrolet Camaro Z28 (Metallic Teal)