I've seen ailerons flutter at low frequency when they were not balanced.  This could be felt in the stick as well and was obvious at 200 mph.

(I didn't buy that plane)

It would take some doing, even for as clever a fellow as yourself, to see the rudder in flight.

Maybe a little close formation could help zero in on the trouble.  Whatever is wiggling may be detectable.

Mark (inverted at the speed of sound - hah!),

 

On the ground, my prop shows .1 ips on the accelerometer at various rpms up to about 2400.  It is balanced at 2600 at  .03 ips.  I suspect some coupled resonant situation.  Today I tried slips (skids?) Trimmed in level flight at 174 KIAS:

 

Left rudder only, ball out to the right, no change in vibration, climb initiated.

 

Right rudder only, ball out to left, no change in vibration, dive initiated.

 

???

 

My "Aeronautics for the Naval Aviator" is not helping me here.

 

Upon landing, I noticed streaks flowing back from the right outboard elevator hinge.  ???  Maybe I finally got all the oil out after lubricating the oversize pins (about 15- 20 flight hours ago).  Of course I was practicing 60 degree steep turns both ways - not bad, less than 200 foot change in altitude in one segment where my mind wandered a bit.

 

I have been thinking of mounting an accelerometer in the tail to see if there is anything to see.  I wonder if your tail stiffener would be a fix?

 

Scott