Transmission/clutch/differential whine

So in the last week my 2003 has developed a whine when it is in any gear at any RPM but only when under slight load and gas pedal is at about 1/4 throttle. My wife says it is more of a chirp – she thought it was like a squeaking fan belt, but more gentle, soft, and sweet.

Happens whether the engine is cold or warm.

Doesn't make the noise when the clutch is pressed in. Doesn't make the noise when the truck isn't in gear regardless of what I do with the gas pedal.

Won't make the noise unless gas pedal is pressed about 1/4, and only in gear under that light load. Press the pedal more and the noise goes away. I can get it to sing and stop, sing and stop.

I don't think it is the differential whine, but mostly because I don't want it to be the differential whine.

Any idea what this could be? My initial thought is that it is a throwout bearing in the clutch, but that is territory I've never worked on, so I'm just guessing.

The truck is a 3.4 liter, TRD, x-cab, with 128,000 miles on it. It is red. I changed all the fluids in the last 1,500 miles.

Any ideas?