1999 Dodge Caravan Dodge Caravan howling noise.

I own a 99' Dodge Caravan that just recently started making a low pitched howling type of noise (like air being sucked or blown through a hole.) Ive checked for a vacume leak and not found any, this would more of a whistle anyway. There are no holes anywere around the air filter or where it conects to the upper intake manifold. The sound starts when the engine is started and all enviromental controls are off so its not a blower. The preformance of the vehicle is uneffected and the tone, volume, and pitch of this noise does not change as the engine goes through its rpm range or shifts gears. Ive taken it up to 75 in fact with no perceptible change in the sound. Ive maintained my own vehicles for years and have never been stumped like this, while its probably something simple that Im overlooking, (fan motor mabey? Exaust system?) Idk, just dont want to get left on the side of the road. Any suggestions?