Problems with nutone ima-323 masterstation

I am currently trying to repair my grandfather's Nutone IMA323 system. Everthing now appears to be working properly except that if the radio is left on for any extended period of time, a mid-range howl goes throught the system. If anyone has any ideas of why this might be, tests I could run (tried some, but won't go into detail unless there is a reply) or where I could possibly find a replacment master station (I know that is a one in a million chance) it would be greatly appreciated.

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