Assuming you mean audio noise you can hear in the CB speaker
What sound does it make
60Hz hum coming from the 120V lines in the wall
Filtered DC hum from the power converter getting in thru the 12v line used to power CB
Or
3600rpm spark plug noise
You don't hear rf interference, RF interference will block / blanket your signal and dampen/reduce what you hear
EMF electromagnetic interference does not have to be in the Radio Frequency spectrum, it just has to be strong enough for circuits to pickup and amplify in the audio output
Our resident HAMS will probably be more helpful
The best way to know what to do to block interference, is to know what the interference noise frequency is