If the whistle appears when the gas pedal is pressed, check the tensioning of the hinged-mounted equipment. At low temperature, compression occurs, and when the temperature rises, the belts are heated and the rubber straps are no more slim. That's why the sound stops.
Also, the cause may be in the brake pads which, when heated, stop "overcharging". Sometimes it turns out that the pendant needs to be lubricated, so when it's cold, it's whistling, and when it gets hot, it stops. The problem is solved by replacing the bearing.