You may have a problem with your Nox
Or, your problem could be lots of EMI.
The Nox with simultaneous multi frequencies is an extremely sensitive, hot detector. I cannot do any kind of testing in my house or in my backyard with the sensitivity over 14 without incredible amounts of chatter. I have both the 600 and 800 and they both act the same no matter what mode they are in. There are also a few urban/suburban sites I hunt that I have to keep the sensitivity around 15 due to EMI. I tried a Gamma 6000, Omega 8000 and a T2 at the same sites and they were unusable, period. Both of my Equinox will chatter under big power lines and around electric fences. I cannot get anywhere near the default 20 at these places. I still get good detection depth even past 12" on coin sized targets with the sensitivity at 15 to 18. Some of the pre-release testers recommended that Minelab set sensitivity defaults below 20 and cap sensitivity at 20 just for this reason.
Jeff