Question 2dof wiper motor jitter/whine

Some years ago i made a 2dof with wv wiper motors, 10k pots, arduino uno and monster moto shields (1 for each motor). I never really used it, but i worked (maybe a bit ;-) )
This weekend i wanted to continue working on it, so i flashed SMC3 on it.
However, i'm running into some problems:
- I noticed that on the neutral position (chair level), there is some sort of whining on my motors. If i push the front of the chair a bit up it seems to stop. Any idea what this could be?

- a second problem is that the movement seems "jerky". When i use the "sine" movement in the SMC3util , its moving, but very shaky. I tried changing the settings (Kp is at 400, but changing it between 200 and 600 does not really help; the other setting do not improve it either , soms seem to make it worse? (eg PWMmin to 200 is worse than 150?)

Would anyone recognise these problems or could tell me how to solve them?

Off couse. The problem is there even with only the SMCutil running, so a screenshot of those settings.
The noice i mostly comming from 1 of the 2 motors. (both have the same settings).
I already addes a .1uF cap (104) across the signal and gnd of the pod and added a ferrite coil around those wires.

the connections between the arduino are cfr the SMC3 manual, except for the pots, they are connected to A4 and A5 (and off course this is changed in the arduino code, also the orignal thing that was linked to A5 was remapped to A1)


I also recorded 2 things:
- motor noice: https://hetgrensland.be/test/2024-11-25 20.05.38.mp4
- the shaky movement: https://hetgrensland.be/test/2024-11-25 20.07.23.mp4

I tried the Rpmw of 25 and there i no real difference (setting it to 5 or 10 generates an extra high pitch sound)

The ground: Normally it should not be a problem (i think) since the construction is made of wood. So there is not metal connectorn betwoon the 2 motors (only possible connection is by the power cables). There is also no metal connection between the motor and the pot (plastic)