Originally Posted by 328X1

Ask your dealer to provide you with the radios' specifications, from the factory Service Manual. It'll tell you what the sensitivity of the unit is. Typically specified NOT as mV [milliVolts], but as microVolts. Any Radio Shack dealer can show you the symbols for milli versus microVolts. A radio only having sensitivity in the milliVolt range is totally deaf. Your unit should have about .25 microVolt or less [less being better] sensitivity rating. You could take your vehicle and do an A/B comparison with another comparible vehicle, as a rough cross-check. Dial in the same stations, etc. You could also use a portable radio scanner.
Any electronics shop, that services radios, can connect their Service Monitor to the antenna (1st point) or to the antenna input jack of the radio (2nd point) and input a substitute test signal, of known parameters. By 'dividing/conquering', it'll confirm 1 antenna issue, 2 cable issue, or verify if the radio itself has developed a problem requiring servicing. (the dealers technician should be able to perform basic tests). The dealer comments sound like pure unadulterated BS to me, based upon the fact I've been professionally servicing radios for over +50 years now. Engine noise can be generated by many sources. Varying high frequency whine, could be alternator related [removing the fan/alternator drivebelt and do a short duration test.] No belt=no alternator noise. Electric fuel pumps, radiator electric cooling fans, etc, pull the fuse for a brief test. Again, no operation, of a given device = no noise source. Other electronic devices have their own unique 'noise/sounds' heard on a receiver. Turn signals, windshield wipers, horns, computers only generate noise while activated, for example. Use a portable AM radio tuned to a blank spot with-in the lower half of the AM band, and do a 'walk-around test', holding it near the vehicle. Symptoms may change as you get closer/further away from the noise source. A bad electrical bonding of the hood can reduce the shielding effect of a properly bonded hood [little difference between when the hood is closed versus if open]. Defective OEM noise suppressors could have developed. Our deteriorating utility systems are notorious for producing increasing amounts of unwanted [and unnecessary] interference. The utilities are required, by FCC regulations, to address noise complaints [don't expect rapid response]. I've only touched on the most common sources. Good luck, and let us know the end results.