If this is just buzz frim the alternator it could be brushes getting short and arcing. Or the diodes are having some leakage and not smoothing the out the ac to dc.

I had a problem like that on my motor home with a GM alternator. AM band just whined with the speed of the engine.

I have a electronics buddy in my R.V. club. He installs radars and radios on boats. He said that the diodes are just crude large power diodes. Some of the auto people buy the cheapest than can get and they get worse with age.

He told me just install a capacitor to the positive wire output marked
"bat." on the alternator.

The capacitor has a positive and a negative leads. Positive on the positive and the negative to the frame of the alternator. Keep the leads as short as possible to help them not to become antennas themselves.

He told me to find a electrolytic capacitor with a value of at least about 2000mfd. @ 50 working volts or larger, if you have the room, out of an old power supply from a computer, t.v. anything that has one that big. Or buy one for about a dollar. It worked a quiet AM band!