Know what it was? A free-wheeling nose wheel! I didn't confirm this until I started hanging video cameras at various places on the airplane--including off the metal step--and actually saw it spinning in the videos! At certain higher airspeeds, because the wheel was not perfectly balanced, it set off some pretty good vibrations felt throughout the plane.
And that's when I discovered "How To Replace The Nose Wheel Snubber" at the Sting + Sirius website (click on "Support," then "How To"). The fix (a piece of rubber fuel line placed inside the axle housing to dampen the spinning of the axle in the bearings) doesn't last long--maybe 200 landings, if you're lucky--but it's worth it for the smoothness and piece-of-mind it brings!
![Laughing :lol:](./images/smilies/icon_lol.gif)