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hello my name is ***** ***** thank you for allowing me to try and help you. well if it were plugs such as not installing resistor plugs going over bumps would not effect it, so rule out plugs and wires. what it sounds like to me is a loose wire or connection at the back of the receiver unit. Is the unit and accessories all OME or original factory equipment? You may have to have the unit removed to check the wiring to it.
Well if your positive with your assumption that all wires and connections at back of radio are in excellent condition then lets go to process of elimination.is this radio and blue tooth all original or aftermarket? first unplug any Bluetooth devises you have hooked to the radio and see if radio still has noise. Then if it still does remove radio far enough to access wiring at back of it. unplug the blue tooth mogul it has a noise filter built into it. Try the radio again if no noise in radio replace that mogul. if noise still persists i would recommend replace spark plug wires , they may be arcing or damaged internally. are the spark plugs original OME replacement parts, if so eliminate that.
then it's defiantly either the spark plugs or spark plug wires . each has resistance built into them . again are the spark plugs original equipment replacement parts and should have R on them with the number, that stands for resistant plugs. If they are then replace the wires they have a carbon core and can over time get damaged or loose it's properties , and the insulation gets hard and cracks allowing arcing either way it's a high voltage serge or arc if you will that's causing it if the spark plugs are as I stated then the wires are your culprit One other thing is the charging circuit if the alternator is faulty , diode problem or internal voltage regulator this can cause a electrical buzzing in the radio as well. you can check this out without a gauge by removing the alternator drive belt and running with radio on. If the noise is gone replace alternator . if noise still there look at the spark plug wires as mentioned. hope this helps.
I never assume anything till I have test results in front of me. if it's more a clicking then most likely wires or one wire when it fires. won't hurt to do an alternator check at that age any way.good luck let us know
close but no cigar, glad you found it , yes they work like wires each cyl has their own coil packs on them. Have a good day