Renix Alternator; Buzzing sound KOEO

So my 89 started making a buzz sound with key on engine off.

With the old hose to the ear sound detector trick, I traced it to the alt.


Now, I got a back up alt. and unhooked the suspect unit and hooked up other alt. (on top of valve cover not installed).

Turned the key on and when alt. case is grounded I can hear the buzzing on the back up alt.

Whats going on? Has it been buzzing the same way for the last 25 years and I just never noticed?

She's putting out proper volts --14.4 about

Anyone been through this?

Ever refresh your grounds like this?

http://cruiser54.com/?p=19

Yes I have done most of your tips.
I'm going to add a ground from alt. to neg. post and see what it does.