Ray -

What type A/C and engine? If you have a belt driven alternator, drive
it with an hand held electric motor & see if you can get the noise.
Might it be a mechanical voltage regulator points chattering (which
they do all the time), but without any of the arc suppression ballast
resistors having continuity on the back side of the regulator, in
which case they could radiate a lot of noise.

A better description of the noise would really help. Can I assume it
is not magneto related? (i. e. have you tried operation on each
mag?)