I'm betting on brake squeal as you mentioned it only happens when it's warm. As I'm sure you remember from school materials expand in heat. Normally the gaps between disc, pads, calipers etc. are very small anyway and if something is wrong (i.e. these gaps are too small) it might cause contact when warmed up. When you notice it next time turn off radio, fan/AC etc., windows/sunroof completely shut (so you can hear better - maybe get somebody else in the car with you?) and start applying brakes lightly (when safe to do so!) - listen for changes in pitch etc. of the "whistling" - if the "whistling" changes/goes away when you apply the brakes then it's 99% brake squeal.