Might possibly be getting some high tone falsing on iron, often this will result in non-repeatable targets and fluctuating TID numbers. Even though I don't own a nox, I see the same happen on the deus every now and again over deep iron targets or iron scattered in the soil profile. You tend to get a brief initial high tone prior to the discrimination fully kicking in after resampling the target on further passes of the coil.
Sometimes it just pays to slow down a bit to give any given detector a chance to properly process feedback from the target, target responses may clean up with a bit of further investigation, if not, possibly time to move on. As an example, some people learnt the hard way with the Deus which is also a fast recovering detector, that a fast sweep speed is not always beneficial, in fact it can cause you to overlook many of the deeper/smaller coins, necessitating in re-covering of old ground again to pick up on those targets, albeit at a much slower swing speed. Anyway, thought it might be something to ponder over.
