Tail Harmonics and Blade Size

K gang, looking for anyone else that has seen this:

Running my D3 up to 3D headspeeds (1850 and 1925) I'm getting a wail from the tail vert fin which seems to go into harmonic vibration at 1850 or better. At 1750 it is smooth as silk. This is with 105 Vic tail blades, (which have been double checked on balancer and are perfect).

With 97mm ms blades the symptom is completely gone and the tail is smooth as silk at all headspeeds.

The wail is definitly from the tail fin as I can see (with the larger blades) it become a figure 8 blur if looking at it from the rear of the heli.

I know G10 is a bit soft and that some of the people I flew with had a similar issue with the X-Specs using the stock 1.5mm carbon fin (which was thin and replacing with a 2mm fin fixed it).

Other than the wail, the heli flies great with the 105s on the tail and authority is wonderful. The sound just makes it awful and it concerns me that some thing is off...

I haven't investigated further but I know the tail struts are good (bolted to lower landing gear base, not side frames) and this heli hasn't even been touched down hard so I didn't bend anything.

Has anyone else seen this yet?