Common mode

Common mode noise is generated between Chassis and Primary.
This is typically noise coupled in the transformer between the primary and secondary winding.
It may also be noise coupled from the drain of a switch FET to a heatsink connected to Chassis.
Or noise coupled to the shield of a motor cable, when high voltages are switched.
The Common mode noise can be reduced by minimizing the coupled paths with Y capacitors or improved PCB layout.
The input filter components – and especially the common mode chokes must also be optimized.