"David Lesher" wrote

Hmm, it should be possible to use a DVM in AC mode on the
alternator output. Fair-good DVM's are really inexpensive.
A True RMS one would tell you in a second. But it would take
some experimenting to say what AC voltages would mean "yes,
you DO have a bad diode."...

Take the alternator off, and take it down to auto zone, and they will put it
on a machine to test the output and the diodes, for free.

Another possibility is that there is a ground loop in the audio system.
(using the frame as the ground to carry the audio signal)
--
Jim in NC