So, not sure if this is going to do the trick, but here's what I would do….
Isolate the power. If you have a 12 volt battery, leaving the radio where it is, disconnect the 12 volt supply coming from the truck and use the battery instead. See if the noise goes away. If it does, noise is getting in through the power leads.
Disconnect the antenna, see if the noise goes away (I think you said it does). Disconnect the coax connector on the back of the radio and see what happens.
I think the approach needs to be tracking down where the noise is getting into the radio and working from there.
You may have to start sniffing around under the dash or in the engine compartment with a hand held radio to see if you can find where RFI is coming from. You may have to get creative with shielding/grounding.