After my 2003 VW Passat Wagon warms up, I hear a whoooo, whoooo as I drive down the road. It seems to get louder around 40MPH, but it keeps the same pace. I can also get this to happen when I’m stopped, the car is in Park and I rev up the engine.
Vacuum leak or plugged air filter. Have someone rev your car while you locate at least the general area of the sound, too many maybes.
Has this vehicle got a turbo?
Sometime one-way valves in the EGR and crankcase ventillation system make noises like that. I’m not an expert on this, but I think there’s one on the cam cover breather hose, another between the crankcase ventilation system and the intake manifold, and another between the exhaust and the intake manifold. (Now it’s time for the experts to take over.)
Thanks for all of the info. I’ll check them out. The car is not a turbo, but I wish that it was.