For the load case 2.1, deterministic waves, I have respected the following conditions:
- AeroDyn and InflowWind = 0
- ServoDyn = 0
- HydroDyn = 1 with H = 6 m, T = 10 s
- Support DOF (platform + tower enabled)
And yet, I do obtain a weird slow frequency in my surge response.

I did not find where this additional frequency could come from. I have tried disabling the tower DOF, it is the same.
Thank you for your help,
Bertrand