It sounds like you have encountered something similar in the past. I would like to explain a few things on the power to get your opinion.
I’ve made sure that the power is clean and a direct connect to the battery. The fuel pump has a good ground as well.
I have checked to make sure interference is received through the coax/antenna by disconnecting the antenna from the CB unit. This is done while it’s powered up and vehicle is running.
The CB unit will go quiet when the coax/antenna is disconnected.
The handheld unit picks this up as well.
This is why I’m thinking a DC filter will not solve the problem. Am I wrong for thinking this?