[Keith Watson]

First dump your pwm values to the terminal window to ensure that they are what you expect.

Long runs of multiple wire cable (like pwm wire, also known as servo wire in the world of radio control models) are known to act as excellent antennas. DC brushed motors are an excellent source of noise for that style of cable. Ways to reduce the noise that is picked up include twisting the cables, adding chokes (magnetic rings) at each end of the cable, adding a filter (capacitors) where the power wires connect to the brushed motors, etc.