While getting ready to leave work today, I noticed a very pronounced electrical-sounding whirring noise coming from the car as I was starting it. The car did not have any trouble turning over and didn't take any longer than usual to catch, but the noise is definitely unusual and hasn't been there in the past.

When I got home (about ten minutes later), I turned off the car for a second and restarted it. This time there was no noise. The car had been sitting for about four hours when I started it after work. I have noticed the noise in the past, but today I had the windows cracked and realized that I wasn't just imagining things.

What might be causing the noise? Is it a sign of something that needs to be fixed or replaced soon? Might the starter motor be getting old or need replaceemnt? Why didn't I hear the noise when I turned off the car later and restarted it?

I've got a few errands to run tonight, so I'll make sure to listen out for the noise then, and I'll report back if there is any change in the behavior of the car.
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'89 244 GL -- 114,050 miles (see profile for info on car)