Particle does not reach steady state sedimentation velocity

Hey there,
I have a minor problem with the sedimentation velocity of a particle. The Particle is located on the top of an tube and shall sediment to the ground of the tube. I am have chosen the following parameters in the couplingProperties file:

modelType "A";

forceModels
(
DiFeliceDrag
//SchillerNaumannDrag
//KochHillDrag
//MeiLift
//virtualMassForce
gradPForce
viscForce
Archimedes
);

I am getting on average the expected velocity (following the Di Felice model ) but unfortunately the velocity is oscillating. The effect descreases, if the tube diameter is increased or the particle is larger.
There is no Particle wall interaction but the diameter of the particles are 5 mm and the tube diameter is 10 cm.

I have added plots, which will show my problem.

A piece of advice would be great

Please let me know, if you need some more information.