With all the boards replaced, it sounds like electrical noise.

Just to clarify the coil suppressors that TKE uses has a resistor in series with heat shrink tubing around it. The purpose being, if the suppressor (capacitor) shorts then no other associated circuitry/ boards are damaged. If the suppressor failed then the voltage drop across the resistor creates enough heat that the soldered joint connecting it melts, which you would not be able to see since it is inside heat shrink tube. Checking suppressors, for me, means pulling on them to see if they separate inside heat shrink tube.

Also its worth noting that another source of noise, is a bad incoming voltage suppressor assembly. This is the black box attached to the incoming 3 phase. There are some of these early on that internal components would break loose from controller contactor vibrations. You check them by unattaching the black box and shaking it. If it rattles, replace it.

Also if this is Synergy Product, I think the UCA board is on CAN channel 1