Methods of filtering noise caused by DC brush motors
The noise could be generated by either the brushes (commutation) or the driver if its PWM.  Easiest fix for brush noise (which would be random or 'static') is a capacitor right across the PMDC motor leads, something like a 1 µF, 100 V ceramic capacitor might be a good starting point.

If the PWM supply is causing problems (something with narrow spectral content), you would need a choke between the PWM power rail and your main supply.