Alternator whine will not normally change with the car in or out of gear; turning on the headlights, the heater blower, or putting on the brakes might make it change. But this does not seem to be electrical since it occurs only under certain conditions.

If it is a whistle when the engine is loaded, I would start with looking around the carb and air cleaner area for some sort of leak that is occuring with high airflow. (Pedal to floor and higher RPM's). Also, check the fan belt to make sure it is tight. That is the best I can make of it with limited info. This is one of those issues where a youtube video/audio might give some better clues.

Ok, the sound coming from the stereo doesn't sound likely. The other question still stands.
Does the sound change when the truck shifts gears?
If it does, that suggests an engine or RPM related noise.
If it stays steady, with only a slight hesitation at the gear change, that suggests a speed or remaining driveline related problem.

Roger