Our 3Dio (FS) has produced a prominent rumble since new. The Bass Roll-Off switched to ON helps but does not eliminate it. The 3Dio also produces hiss with irregular intensity, but we have not found a pattern to it.
The rumble and hiss are present on all our recording devices: A Sony A6300 and an AX-53 camcorder, a GoPro Hero 9 w/Media Mod, and a Zoom H6. We have tried three different TRS cables, different brands of 9V battery, and many different mounts. The rumble and hiss never go away. Both increase with gain until the recording device's self-noise overpowers them.
We thought the problem might be environmental, but we just drove four hours to record in the middle of nowhere and the rumble and hiss are still present. Even after filtering in post, I don't think the audio quality is good enough for a finished product. Please help.