Thank you all for your replies.
Based on your answers which led to further research, I think this nails it:
If a starter solenoid receives insufficient power from the battery, it will
fail to start the motor, and may produce a rapid clicking sound. The lack
of power can be caused by a low battery, by corroded or loose connections
in the battery cable, or by a damaged positive (red) cable from the
battery. Any of these problems will result in some, but not enough, power
being sent to the solenoid, which means that the solenoid will only begin
to push the engagement gear, making the metallic click sound. Starter
solenoid problems are best diagnosed by an experienced auto-electrician.
I previously referred to a "grinding" sound but it would be more accurately
called a "rapid clicking sound." My bad.
So, other than waiting for the generator tech who is swamped with work, who
would be the next best person to fix the issue: electrician? auto