You need to isolate the cause of the whistling noise as to whether it is the engine, the drivetrain, or wind noise. To see if it is engine noise, put the car in neutral and acclerate the engine to see if you hear the noise with the car not in motion. If you do, the problem is most likely a vacuum leak somewhere. If it is just a vacuum hose, the cost would be minimal. If it is an intake manifold, the cost may be more expensive.
If you don't hear the noise when the car is not in motion, find a quiet road, take the car up to 40 miles per hour and shift to neutral (but leave the engine on). Does the noise come about 30 miles per hour and then go away at 25 miles per hour? This isolates the noise to either the drivetrain or wind noise. At this point, have a garage raise the car up with the wheels off the ground and accelerate the car to 30 miles per hour to see if the noise is there when the wheels are turning and the car isn't moving. If it is, the problem is someplace in the drivetrain.
If the problem is isolated to neither the engine nor the drivetrain, then there is some body trim piece tha is causing the problem. THis is harder to find, but usually simple to fix.
I had a simlilar problem with a 1965 Rambler Classic that I owned. THe noise would start about 45 mph and top at 50 miles per hour. I determined that it was something in the grille and decided to leave it alone--served as a audible speed indicator. However, I took it back to the Rambler dealer for wsarranty work on something else. The service manager road tested the car and he decided to fix the problem even though I hadn't complained about it. He put some body putty around one of the upright trim pieces on the grille and it stopped the noise.