get a buddy over and jack the truck up in the rear and one person bring the rpm up to that point while the other tries to determine where the wwhistling is coming from. Being you can hear the whistling from inside the car tells me it should be pretty easy to find as most engine noises like this arent usually noticable while driving. Being you can also hear this while driving I would also check any vacuum lines you may have in the truck that may go to the wiper motor if of coarse you do have vacuum wipers. Another possiblity is a T/O bearing or pilot bearings also will make this type of noise, I have also seen people leave a little extra on the cotter pin on the front wheel bearings that it will rub up against the fgrease cap and cause the same noise, as stated above being you had different gears the engine was considerably loader and you never noticed it.
