Sounds to me like an issue upon startup and the RPM's settle in that the regulator isn't breaking over into regulation for a brief period. This could also be linked back to the alternator. Until it reaches the needed RPM to satisfy all of the rectifying diodes and put out enough stable voltage and current the regulator can't settle in. The whine noise is usually the sound of AC voltage coming from the alternator I believe. The diodes in the alternator are what converts the AC to DC. Very dirty DC since it's not filtered but DC none the less. You may be perfectly fine if it's just a momentary thing on startup. Just sounds to me like it takes a second or two for the trucks RPM's, the alternator, and the regulator to all come together and break over into regulation. Someone else might be along with deeper knowledge than me to point you better to eliminate the whine completely.
