For a moment after the engine starts, I hear a squeaking / creaking sound. My fairly uneducated guess is that the alternator is making the sound, but I don't know exactly when the alternator begins to spin.

If the car has not cooled down completely after running for a few minutes at least, the sound does not happen when I restart it. Therefore, I suppose that either heated lubricant -- or just the warmed engine oil -- plays a role.

MY99 TS 2.2 w/ 183000km

does it make it for a while? like a continuous squeeking?

or is it like one single "SQUEEEK!" and thats it....

maybe a belt slipping? how do your belts look?

It's only a momentary high-pitched creak lasting nearly a second. The belts are all fine; the timing belt was replaced last year. Starting the car while standing at the engine, I noticed that the noise came from near the throttle body instead of the alternator. The source was toward the back, but lower down. Oddly, it happens AFTER the engine starts completely, not before. Again, this startup noise does not happen if the engine is still even slightly warm.