Based on Doug:
I would try to match the moog preload, which is about 0.6A.
With the present current limit of 4A the preload would be 15%

The idea from Doug:

My theory is that the system has a CG out of balance. The torque required to overcome this out of balance is similar to the torque required to overcome the bearings. If you move in the direct of the out of balance the two affects cancel and very little torque is applied. If a drive is in the zero torque phase it becomes unstable as a result of the gap in the gear teeth. If you move in the opposite directly the torque required must overcome both the out of balance and the bearing. This effectively preloads the drives.

Prepared the test case.

Since the position filter is changed, we found the vibration happens at -1.5 deg to 0 deg now. After we tested the preloading of 15%, we did not see the vibration between 1.5 deg and -1.5 deg. Now, we are intending to try more large range of movements.

The comparison of currents between MOOG and Tekniker are:

Please help to review this ticket. Thanks!

Thanks for your support Te-Wei! Great, fast analysis. Look forward to get this solved!