[TenTec] Digital noise in Omni 6+ audio

[Barry N1EU]

There's a fair amount of low-level objectionable noises (high-pitched
tones, tuning encoder noise, etc) in the Omni 6+ receive audio.  It
detracts from what is otherwise a fantastic sounding receiver.  Using the
LP function gives some, but not complete,  relief to the high-pitched tones.

Has anybody investigated/analyzed this to any degree?  I would think that
the noise must be introduced where the IF/AF board audio loops to the CODEC
(via cables 54/56) on the logic board for dsp processing.  I pulled the
logic board and was surprised to find that the CODEC chip's "analog" ground
U11-17/18  is isolated from the rest of the logic board and is only
connected to the shield of cables 54/56.  Yet the shield of cables 54/56 IS
connected to chassis ground back on the IF/AF board.  This just leaves me
scratching my head - has anybody else looked at this circuit or can
otherwise comment on this grounding scheme?

I also looked at the +5V supply to the CODEC chip U11 and no glaring issues
jumped out at me.

73, Barry N1EU