Whistle noise I have a whistling noise coming from the engine, Notice when accelerated over 50 mph, quites when under 40 mph. Doesn't matter if air or heat is on or off. Doesn't make noise when turning. Can not pin point exactly where its coming from. HELP!

Leaking Vaccum Line or the intake system has a leak between the air filter housing(air box) and the engine's throttle body, that's usually the culprit!

yes could very well be an exhaust leak.but that should be kinda loud and annoying..also u could have a slight whistling sound coming from ur air intake hose coming from air cleaner box to ur intake or throttle body or ur air inlet goin to the motor. when the motor is accelerated it would produce a high pitch sound like a whistling .hope this can help

The first thing is to make sure you are not low on transmission fluid. I would expect more a whining noise than a whistling. But the slower shifts may definitely indicate a low fluid level and/or fluid that is long overdue for a drain and refill.

check for a manifold gasket / pipe leak

Did you remove any plastic/metal splash shields under there to get to the oil plug/filter?
Make sure you put those back in correctly as they could catch air and make noise if put in wrong.

Other than that, I can't think of anything oil change related that would cause a whisteling sound.

Sounds like a vac leak, could be the power brake booster or a hose.

I don't know how loud the noise is, but its weird that you mention 50 mph. At that speed or above you are keep a constant rpm level. I say this because it might just be your air box loose and the sound of the intake air is making it. A 2008 truck should not have any major problem. Sometimes an alternator will sing or whine but never heard anyone say something is whistling. One other thing is sometimes for some reason ive heard a noise like that coming from the speakers of my car that goes up and down with the speed of the motor, but that was when I was tinkering with speakers and sub outs and amps. Thats all I can think of.

I had a problem with a whistling car after 50 mph and it turned out to be the front window gasket. I put silicone on the trim and problem solved.

try and look at the belts, if they are wore out replace them