2010 Whistling Sound

Hey guys, back again. I posted a thread a while back about a whistling sound in the cab of my '10 Tacoma DC 4x4 TRD. Upon further inspection and trial and error, I have deduced that the whistle is caused not by wind noise but rather something in the drive train perhaps. It has the same pitch as, say, a muted referee whistle or a cricket; just not as loud. It starts around 25 mph, doesn't change pitch as I accelerate, and seems to get louder AS I accelerate. It quietens down if I let off the accelerator, but doesn't go away. Simply put, it chirps when I accelerate and quietens immediately upon deceleration without fail. The dealer won't touch this issue, so I thought I'd bring it before you guys, the real experts. If anyone has any idea what causes this, I will be forever grateful because it is driving me and my passengers MAD! If not, I see a trade-in happening soon...

Thanks again.