Feedback’ sound from FSII-Base II 4-wire ports into intercom Matrix:
When a FSII beltpack user walks into the control room were the intercom panels are located with their talk button latched, feedback may occur (normal acoustic). 
Occasionally (can be months apart) the feedback will persist out the 4-Wire ports even after the Talk key on the FSII beltpack has been unlatched. 
The feedback can be heard on the intercom panels. 
The feedback cannot be heard on the beltpacks. 
Unplugging the cable on the FSII 4-wire port stops it from being heard in the matrix system. Reconnecting the cable will reintroduce the noise. 
Power cycling the FSII base stops it.
Solution:
4-wire ports should not feedback on their own. We’ve seen the same symptom puzzle people once before, and in that occasion it turned out to be that the 4-wire was linked to the 2-wire ports via a channel. The 2-wire port was not terminated and not nulled, so reflecting. Please check the configuration with the customer to exclude this.
Also  make sure the 4wire cable between the FSII base II  and the intercom matrix 4wire port do NOT have pins 1/2 and 7/8 connected (on the FSII Base II side), only pins 3/6, & 4/5 on the FS II Base II 4-wire port should be connected