Whistling AC - 2008 Chrysler T&C - Only at certain RPMs

After almost a year of fruitless trips to my local service center of a national used car dealer, I am still having a problem with an in-cabin whistle. This is the problem: There is a whistle inside the cabin of the car when: 1) The AC compressor is on, 2) The car is under slight acceleration, and 3) The engine is running at one of the following RPM ranges: 1500-1800 RPM, 2500 RPM, 3000 RPM. It will happen at any speed: pulling out of a parking space, or running down the interstate. On a long trip, the whistle will occur every 45-60 seconds while on cruise control, as the car slightly accelerates to recover speed. I have read that it is likely the AC Expansion Valve (two other posts on CarTalk), but my question is Why the dependence on the engine RPMs? I can make the car whistle continuously if I run the car at 2,500 RPM in 5th gear on the interstate. At 2,400 RPM or 2,600 RPM, however, the whistle disappears.
The used car dealer can reproduce the problem on the road but not in the shop. Any ideas on how to reproduce the problem in the shop, or any explanation as to why it is dependent on engine speed would be welcomed.

vacuum leak ?

  1. We’ve replaced the expansion valve, no change.
  2. We’ve sprayed the fuel injectors while monitoring the mixture ratios, no mixture change.
  3. We’ve disconnected the radiator fan, no change. (And we’ve reconnected the radiator fan.)

The car still whistles/whines.
However, when all the refrigerant leaked out of the system, the whistling / whining stopped. (The new expansion valve failed, and the local car repair place replaced it as a warranty fix.).

So, we’re still stumped as to the source of the problem.

If it was as simple as a vacuum leak, then we could reproduce it through some other means. But it only happens a) when the AC is running, b) at certain engine RPMs, c) when there is refrigerant.

So, the used car dealership is telling me to take it to the car dealer and have the care dealer work on it under the extended warranty. I just don’t have any confidence that the Chrysler car dealer can fix it any better. The only advantage that they have is that they can put in a part, and if it doesn’t fix it, they can put the part back into inventory.