If you only hear the noise on AM it is likely being picked up by the antenna and is radiating through the air. If you also hear it on FM it is coming in through the power wires to the radio. The first thing to try is to unplug the small wires on the generator. If that solves the noise the generator likely has one defective diode of the six. That will cause excessive "ripple" in the output voltage which will be heard as a high-pitched whine that changes pitch with changes in engine speed. A professional load-test is needed to verify a bad diode. Maximum output current will be exactly one third of the generator's rated value.
For other sources of noise you typically would need an oscilloscope to look at the voltages on the power and ground wires to see where the interference is coming in. Without a scope, you can try a new ground wire between the radio and car body as a test. With a bad ground wire some radios will still work by finding an alternate ground path through the dash light circuit or tail light circuit. Also look for corroded or broken ground straps between the engine and body, hood and firewall, and various places like that. Other circuits unrelated to the radio often rely on those ground straps. With bad or broken grounds the current has to find an alternate path and that can set up current flow in other wires that DO relate to the radio.
If the noise is only heard on one side of the car or in just one speaker, check those speaker wires for voltage with a digital voltmeter. All of them should have close to six volts. If one has 0.0 volts that wire is grounded and is bypassing part of the amplifier circuit in the radio. That usually leads to failure of that part of the radio if it is allowed to play that way too long.
Thursday, July 25th, 2013 AT 10:35 AM