whining from dash

have a whining noise coming from the center of the dash on a 1993 buick lesabre limited, it starts immediately upon ignition and raises and lowers with the engine rpm's, it sounds like a vacuum leak but I don't know were to look. You can hear it in the front seats the best, passenger or driver seat. When you listen with the hood up you can barely hear it towards the fire wall, but nothing obvious, doesn't seem to disappear with higher accelaration and doesn't change at all with the heater or air conditioner settings in any position. just raises and lowers with the rpm's. makes the same whine, if you put it in neutral while your driving as it does in any other gear or idle. driving me nuts

any suggestions will be appreciated

thanks in advance