So I noticed the other day (and figured it must have been something other than the car making it) that the car made a "whistle" at idle sitting at a red light. Both times I had just driven about 20 miles, and said redlight was my first stop.

The sound lasts about half a second, is not terribly high-pitched, but does climb in pitch towards the end of the sound.

Anyone have any ideas? It's only happened twice.