Need help to diagnose noise in intercom

[Albatros]

As, mentioned. Ground loop. It creates noise and every cable on that ground acts as antenna and pick up noise. It has to be grounded to same ground bus as your radios. Make sure you not picking up ground from frame, mounting plate etc. by accident. 

[Jerry 5TJ]

On 12/17/2017 at 4:47 PM, Geoff said:

Found the culprit but not the solution.  The noise stops when I unplug the panel docked 696 from ships power. 

Any ideas on why it only makes noise when plugged into ships power?

(1) If the 696 audio output ground is common to its DC input power ground that is a potential ground loop defeating your intercom’s single point ground.  

You might AC couple the 696 audio output ground with an unpolarized capacitor.  

(2) If your 696 is wired to your panel mounted GPS with a serial cable that cable’s ground may be a source of loop, too.  

Thot: Get a FlightStream and let the portable communicate wirelessly.  No ground loop that way.  

ASIDE:  These audio issues are sometimes tricky.  My present GMA350 audio panel has clean audio — unless I touch the CB subpanel, which causes a very slight whine in the headsets.  In that case I’m the ground loop.  I’ve been called worse.