A whistling noise indicates a vacuum leak.
The most common cause of the whistling in a 2003 Impala is a vacuum leak. Any of the lines under the hood could be causing the noise.
The head gasket is about to blow.....Typical ford..
Sorry forget to mention it in the motor.
Your brake booster is malfunctioning
If it it shutting off because of the hissing, it is likely moisture getting at electronic parts and shorting them. The hissing would be caused by cold water contacting hot engine parts. If the hissing is only heard after YOU shut off the car, it is likely a small leak in a vacuum hose. These are usually easily fixed.
Check your steering pump to see if you need to add more power steering fluid.
I would think that you have a leak in your exhaust system.
Brake booster problem?
wind interaction with the stiff radio arial ?
Vacuum brake booster is defective or the hose going to the booster is leaking.
A vacuum leak usually from the intake manifold gasket or brake booster hose/connection will cause a whistle upon partial acceleration. It will usually go away when your foot is off the gas or when more throttle is applied.