Definitly repeatable on the ground.  Most obvious when ATIS is beaking the com squelch, but no ATIS voice.

 

When 3rd crew audio panel is unplugged, noise goes away.  Pretty repeatable.

 

When PIN D is pushed out of the 3rd crew audio control plug, problem goes away.

 

Problem solved itself after 2 hours of troubleshooting.

 

 

Figures.

 

 

 

 

Later , pin d and another pin(ics) wer found swapped by the dom.

The squad has now morphed:  RadAlt (EGPWS?) audio very low.  There is still a hiss after ICS squelch break,and after ICS voice ends. Same with ATC audio.