What causes a car to whistle when you accelerate?

It could be caused by a leaking or disconnected vacuum hose.

additional info: assuming this whistling noise is coming from the engine and not from the wind whistling through some bodywork I'd recommend you open the hood with the engine running in the driveway and run the engine up and down (with the linkage by the throttle body on top of the engine or by having someone in the car press on the accellerator) and listen for that noise. I believe that a vacuum leak that you could hear in the cab would be serious enough to drastically affect the operation of the engine and you would notice surging, hesitation, dying, poor fuel economy etc.

This further diagnosis may point you in the correct direction to get this resolved.