Noise in steering column

Our 2002 Chrysler Town & Country minivan with 42K miles has an occasional humming noise that seems to be coming from the steering
column. it is not audible once i step out of the van or when i raise the hood to listen. Steering remains normal and turning doesn't change the sound, but the pitch does change when I accelerate. The sound usually begins after driving for at least a half hour and then stopping at a stoplight. I haven't had a mechanic look at it yet. I just want to ask first if that sound is something to be concerned about, or what to expect if I have a shop look at it. I would guess it would be hard for them to duplicate it.
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