[Thumper]

Starter button (stuck) would be a candidate. Unfortunately, the connector for the starter button is buried under the gauge cluster. Disconnecting it could stop the perpetual start! But it could be the solenoid. After you recharge, verify that the starter isn't constantly engaged (turn on the key!). Tap the solenoid with a screwdriver handle multiple times (with power turned off). This may break loose a stuck solenoid. Turn the key on again. Did this fix it? Get a new solenoid.

These are the two most probable issues (starter button, and solenoid).