[Bob Putnam]

Typically, when you get a buzzing or chattering from a current draw, it's the result of high resistance somewhere in the circuit. It could be anywhere from the battery connection to the starter solenoid ground. I would:

Run a wire directly from the battery positive to the S terminal on the solenoid. If the engine turns over, the problem is likely within the ignition switch circuit.

If you still get buzzing, the problem is likely the starter solenoid ground.

A third possibility is that the cable from the starter solenoid to the starter is shorted out.