Old Farfisa with DM02 - Noise on speaker phone

[Krobar]

This install is 5 years old and the speaker phone never worked. 
Today I found out that is because the Voice and MIC form the DM02 was not connected to the main PCB so it now at least works. T
There is a fairly loud constant buzz though between the door station and all keypads (Keypad to keypad audio is good). The cable is a heavily shielded cat7 and direct wired so should not be an issue I think. I tried removing J5 which just seems to make it louder. The wiring looks to be the same as page 8 in the DM02 manual and J1 is set 2-3. There are 3 spare wires as the grounds are all just connected together on the door station as are the two 12vs. My DM02 does not seem to have a J6. Any tips on how I can reduce the buzz?
Hmm, disconnecting the DS Speaker and Mic from the DM02 does not stop the buzz at all. The DM02 is in the middle of a JP8/JP8A module daisy chain, could this cause buzzing?

[slychiu]

Low frequency buzzing is normally caued  by mains interference through the wires.

This could be the speaker and mic wires from Farfisa to the DM02 or from DM02 to the Comfort 

I woud start by disconnecting the Farfise from the wires and connecting it direct to DM02 using short wires to check if the buzzing disapers That would ean the interference is on the farfisa wires in the wall