There is a good reason I ask this.
I normally hunt a fairly clean site in Auto-tune, then flip to 0 Disc. to ID the target.
A normal target, in Auto-tune, starts out soft and low, with the pitch and volume both going up (like the pinpoint), then dropping off as soon as the center of the coil makes it Closest Point of Approach to the target. A really big target does the same thing, but blasts your ears off, then gives you the "bell tone".
Last year, after a big storm came through, I was searching some tree rootballs that were exposed where numerous trees had come down. I was hunting in 0 Disc, and got a really funny signal on one of the rootballs. It bounced around, giving me all three tones. I couldn't pinpoint it. It seemed to move around, sometimes up to 9" in any direction. On a whim, I flipped down to Auto-tune on my CZ-5 and got an interesting surprise. Instead of a target that sounded like what I described above, the sequence of sounds was completely reversed. I would get a belltone, then the target would hit loud and high, with the volume and pitch dropping off as I moved away from it. I had a "negative" target, which is a kind of hot rock. Since I was getting the belltone, I figured I had a pretty good sized one. Sure enough, I managed to isolate the signal to a chunk of stone about the size of two softballs.
I've been finding lots of these "negative hot rocks" in the past couple of years, since I moved here to MI. One thing that I've found, is that while small negative hot rocks usually won't make a peep in 0 Disc, large ones will, but will only sound off several inches away. Once you hear one in Auto-tune, you'll know it, and it doesn't sound anything like the "positive" metallic targets that we're searching for.
Another reason I like to hunt in Auto-tune, is that around here, there are powerlines overhead just about everywhere. This results in a lot of RFI that will cause extensive falsing in 0 Disc. However, in Auto-tune, the same RFI sounds more like a buzzing, staticy sound.
A coin on edge or a nail will give a double-beep, and in Auto-tune, also gives a double-blip or sound. Several smaller targets (pieces of small wire, etc.) will give a "fluttering" kind of sound in Auto-Tune. And, as I mentioned above, a "negative" hot rock gives just exactly the kind of signal as a positive target gives, like when you're GB'ing your CZ with the bobbing method, and the sound hits as you lift the coil away from the ground (negatively-tuned).
Again, I ask, what does your mystery target sound like in Auto-Tune?
HH from Allen in MI
"> I did find 3 IH pennies and 4 wheats but they all repeated well. Brown clay. Coins don't sink far in that stuff.