Are you talking about picking up a "whine" on AM radio, or after-market amplifiers? I've experienced whine when there is a grounding problem ... energy is being radiated by the positive wiring, but not being absorbed by the ground wiring, because there is high impedance in the ground somewhere.
Use an multi-meter set to ohms and test the resistance on your 7-way cable's ground pin to the trailer frame, and on your truck's 7-way connect ground pin to the truck's frame. corroded connectors or a lose / broken wire can cause high impedance.
some folks say the trailer grounds through the hitch... if there's too much lube or paint in the way, you'll have a high impedance path there as well.
