How to "clean" a DC brushed motor : noise on CANbus even without moving!

We have very odd problem with a DC brushed geared motor that we are driving with a classical H-bridge chopper (20kHz PWM at the moment). The motor kinda looks like a Mabuchi from an R/C car but with an epicycloidal gearbox in front of it.

We had another motor from another manufacturer that was working perfectly but with this one the CANbus keeps dropping frames as soon as it is even connected with the speed controller. Not even moving! Just connected with no current whatsoever.

We’ve tried various controllers and also various motors (same model) and the problem doesn’t change.

We’ve also recently seen some problems with low voltage signals picking up noise when it’s connected.

The only fix is to use a separate CANbus AND power supply. In all other cases, problems keep on piling up.

We’ve put different toroidal chokes and went up to dozens of turns into big ones but while that helped, definitely not enough…

Any idea or lead that we could follow?

Thank you all by advance,