Nutone IMA-4006 buzzing sound, help!

I live in a house that is 22 years old, and it has a Nutone IMA-4006 music intercom system. It was working fine during the year I have lived here until this fall one morning around 3AM, when I awakened to an annoying buzzing sound on my intercom that will NOT go away unless I turn down the volume all the way - but I enjoy using my intercom! Please help! How do I fix this? I see other people have this problem?

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