Assuming it has, the first and easiest check is to reduce sensitivity to 5.5 or 6.0. That should be low enough to eliminate falsing due to circuitry.
Next thing is to move to an area where it worked correctly before this began. Does is still chatter?
If it still chatters, and worked in this area before at low sensitivity, I'd want to change coils next.
If you have no other coil, Mary can probably help you.
I'd be inclined to think the coil got water inside, or the internal coil wiring has come loose (if that's possible). Either way, it's probably time for another coil.
If you just changed batteries, maybe you had the sensitivity cranked up to get the very last chance of detecting with the old set of batteries.
Make sure you set the right battery type set in the detector also, though I think that just effects the charge rating on the screen, and not the detecting circuits.
Best of luck, and be sure to let us know what the problem was, once it's resolved.
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