回复：annoying car problem

It is may be your break come to life. When it is time to replace the break pads, the pads starts make whistling noise due to a build in metal spring folk on the pad. When the car is drived for some time everything starts to get heated up, the pad's spring folk may just expanded to touch the break disk thus makes noise, on high way, everything gets cooled down due to high air velocity.

Certainly there may be other causes, it is hard to tell for the limited given description. Just my two cents.