Sqealing problems

I recently replaced the intercom system in a house which previously had an "Aiphone" intercom system. The original cable which I have re-used is a 22 conductor sheilded cable that was looped at each station and hence I was able to bring each stations 3 wires to the master station as I was only replacing 7 remote station system. My problem is that the system "squeals" when the door answer button or the inside talk buttons are depressed. Any help would be much appreciated.

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