2003 Chrysler Town and Country Noise

Our 2003 Chrysler Town and Country has been making a high pitched whining noise. It sounds like it is coming from underneath the front of the car. It is high pitched and sometimes gets pretty loud. The pitch is dependent on the vehicle speed. It is intermittent and tends to occur during the first 15 - 20 minutes or so of driving. After about 20 minutes, and especially after highway driving it tends to diminish and eventually is not noticable. We have mentioned this to the dealer a couple of times, but they have taken it for a drive and claim to not have heard anything.