I had that happen on my Audi; the return spring in the ignition switch had broken, and the key was not returning by itself. As a result, the starter motor was not switched off.

The humming I heard was the starter motor going for a ride!

My short-term solution was to help the key back to the "ON" position from the "START" position.

The long-term solution was replace the ignition switch.

Engine and starter are still doing fine!

Good luck, and good hunting!

BCingU, Jim

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'96 E300D 60k mi (wife's daily ride)
'95 Audi 90 120k mi
'92 GMC Suburban 139k mi
'85 300SD 234k mi (my daily ride)