Grossgary, thanks for your responses.
Vibration is not quite the right description of this sound. It's sharp, distinct, rhythmic, and steady--not loose, like I think of a vibration. And no, hard acceleration does not produce more of the sound than mild acceleration.
No, not me, but I bought the car a few months ago (@ 125,000 miles), so I don't know prior to that.
Same answer.
Lars