Get yourself a 2500 ufd, 50VDC capacitor and tie one lead to your power wire leading to the radio, the other lead to a good chassis ground. What you are hearing with the key in the "on" position is the electronics, (ECM) that let the engine run. The shielding for RF energy is terrible in the automotive world and the spurious noise you are hearing is resonating through the truck's electrical system. Noisy, low buck components contribute to the phenomenon as does the alternator magnetic fields. There is not very much powered up when the keyswitch is in the "accessory" position, mainly the engine ECM, and the alternator charging circuit. This noise is easily "shorted" to ground potential with a capacitor. When you tie one leg of a cap to the electrical signal and the other to ground the spurious noise will pass through the capacitor allowing a "smoother" DC voltage to pass to the radio power supply. By law consumer electronics have to accept any outside interference of RF energy or be licensed such as powerful radio transmitters. A citizens band radio is limited in it's power output and therfore immune to government intervention. An automotive "noise filter" that you would purchase at a stereo store is nothing more than the above mentioned item in a fancy case. If you can't hide the capacitor out of sight, these a good way to go. Don't eat Other Dogs' mushrooms. I hear he is looking for test subjects. Rob