just want to share my experience and a little guess. I had the same problem with my 'hand-made' audio-DB9 cable. The problem went away after I connected DB9 connector's pin9 (GND) to DB9 connector's shield . the root cause may actually come from USB-Serial cable - I guess, its serial side connector's shield is well grounded, but its gnd signal (pin 9) may not be well grounded.
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