You say the truck is 12V, what provides the charging, alternator or generator?
Try this test but only long enough to see if the noise is present or not. I'm talking seconds not minutes.....
Disconnect & remove the fan belt, start engine turn on radio and listen.
If no noise, the source is the charging system, troubleshoot accordingly.
If noise remains, it's not the charging system, continue troubleshooting the ign system.
Good luck
