Running X-Plane 11 and just purchased the C182T.

I'm getting this strange problem where the aircraft nose yaws left then right in a constant repeating pattern while I'm flying straight and level.

It is not caused by my flight controls as external views confirm there are no rudder movements.

Wind is also minimal and this happens even in direct head or tail wind without crosswind components.

Any ideas?

I haven't got that specific aircraft, but I do find the reaction to turbulence in X-plane is excessive on the yaw axis. Some aircraft developers seem to mitigate this to a reasonable level, but I definitely notice it with the default aircraft. Flying in real life I find turbulence affects roll more than yaw and of course there are the plain old bumps when flying into subsiding or rising air. These often have little effect on any axis, but you sure feel them, something difficult to replicate in a desktop sim.