Do you have a multi meter? If so check the battery voltage. A charged 12vt battery should have 12.6 volts on it. A 12vt battery showing 12vts is run down. Next check and see if with the key OFF the starter is getting voltage. If so, how much. It should not be getting any. If it is your solenoid is stuck, or burned closed.
When you heard the noise was the starter trying to turn the engine over? If the solenoid is burned closed,it is possible that a low voltage is all that is getting through, just enough to run the starter slowly, but not enough to kick the gear up and in.