Huh. Maybe it is the keel. If there’s slack between the keel and the keel pin I suppose the turbulence could cause it to vibrate.I didn't feel any vibration in the tiller handle. Just in my feet. Now the keel is pitted so it could be that the keel is pitted heavier on one side than the other.
But I think your keel is like mine, > 500 lbs. Hard to imagine a big heavy lunk like that vibrating much.