{Collins} R-390a hum *within* the audio module?

My apologies if this is only a question because of my inability to do a
google search with the right keywords, but everything I've found so far
regarding an R-390a with hum from the audio module refers to a hum that's
actually heard in headphones or speakers -- I'm trying to identify why the
module itself would hum loudly--if it's something to be concerned about and
if so, any suggestions on where to start.

As far as I've been able to determine, none of the outer components are
vibrating and causing the humming sound...if you've ever used a cheap
charger on your car's battery, it's about the same loudness.


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