Yes the residuals look like you described. I don't know about "high" frequency though. The fluctuations look pretty much the same in amplitude and frequency like the wobbles in the Cl plot. 

I remember reading in the Fluent user's guide that a too small time step can cause numerical instabilities at free slip walls. Unfortunately I can't find it anymore. I guess this is what is happening here. The pseudo-transient time step was 0.001 s which corresponds to about 2% chord length traveled per time step at my 20 m/s. I can try again with a larger time step and monitor the pressure like you described. But I remember from previous simulations that I often got divergence for larger time steps.