Yesterday I was driving my Insight with the radio off (a rare occurrence) and noticed what I can best describe as a faint high pitched humming sound only when the assist is full. It sounds like it's coming from under the dash but it's hard to tell. I have never noticed this during the 3 years I've had my car. It's a manual 2000 with the original battery and nearing 69000 miles. I don't have any noticeable problems with the car, and Honda did a courtesy check on the 144V last time I had it serviced and everything checked out fine. Is this normal?