2003 Volkswagen Passat Whistling sound

Would you say it sounds more like the buzz of an airplane engine? If so, a good suspect is a noisy front wheel bearing. Your mechanic can identify that by listening next to it with a stethoscope while it's running in gear, on a hoist. Sometimes you can also feel it by spinning the tire slowly by hand and feeling for the vibration on the coil spring.

If it's really a whistling noise, check for loose hose clamps on the fresh air tube, and anything else related to the turbocharger.