If you can identify the source of the noise, it doesn’t really matter how it’s getting in. What matters is that you can turn off the source of the interference.
If it’s not possible to locate the source of the interference, or not practical to turn it off (a pacemaker, traffic lights outside, passing taxi cabs …), then we need to look at trying to make the system less sensitive to it. Best if you can isolate the source and turn it off.
Has this noise always happened, or is it something that started recently?
If it’s not always happened, try to think of what’s changed.
Why are you using the FetHead? Audient say that the iD14 has “World class mic pres”, so I’m wondering why the FetHead is necessary.