I have an odd noise (whistling, not like a whistle being blown but similar to a tea kettle whistle, or high pitch squeal) coming off my car while accelerating. It is most pronounced when I first press the gas lightly and goes away when I press the gas hard. If I keep my foot lightly on the gas I can keep it going indefinitely. Noise is not present in idle.

It does seem to more dependent on RPM rather than speed - as it will whistle at high and low speed. The noise is present in park, neutral, reverse, and drive. In park, I was able to rev to high RPM - and the noise increased in pitch starting at about 2000 RPM until 4000 RPM or so, but seemed to go away after that (although I didn't go much further than this).

Recently it seems that the noise will remain present when I first take my foot off the gas - sort of like a spinning that then stops.

At startup there does seem to be a brief very high pitch noise but that may be unrelated.

The sound appears to be coming from the passenger side engine compartment near the alternator and belts - although I have not been able to determine exactly where from. I have read a little about a PCV valve which is close by that seems to make a high pitch noise when beginning to fail, but not sure if that is it.

The car is a 2005 Buick LeSabre with 105k miles on it. I recently (I think November) had the transmission fluid changed, but no other work since then. Transmission shifts pretty smoothly while driving and no jarring, no slipping in and out of gear, etc.

Is belt tight moving in a straight line, or flopping around a bit while squealing?
Often the squeal is belt slipping or even a bad bearing on one of the accessory pullies, even tensioner/idler pulley for instance.
A visual check while squealing, to ensure they are turning properly at the right rpm, without seized or wobble may point to what'* up.
Often an auto stethoscope will allow you to pinpoint the noisy one more accurately.

Check the PCV. If someone replaced it they may have left out the lower O-ring and/or spring that holds it in place.