If I sweep real slow or turn the target slightly while air testing. But a quick level sweep back and forth generally produces an even consistent tone unless more than one target is in the hole, something is on edge, or a up side down bottle cap, bi-metal target or other unusual non coin shaped item. Honestly, your best learning procedure is to stay away from the confusing sounding targets and swing until you get a clean, repeatable signal, after you do that for a little while it is much easier to start guessing which of the questionable sounding ones are worth digging because you will recognize which tones in that mass of arpeggio are from good targets and which ones are falses. Situations will repeat and you will start to stitch them together into a firm understanding of what you're hearing. On some days according to moisture level in the soil, mineralization of the soil, my machines can give less than perfect clean sounds even on medium depth coins with no trash around, especially in freshly dug up, air entrenched soil. Generally though, there is a tangible reason even if at that very moment, I don't realize what it is. A good example would be a farmers field he dumped fertilizer on just before it rained. Bench test some coins, see where they register on the smart find, start to recognize their tones and the quality of those tones (not crushed or scratchy) but round and smooth. Now, go find a not to trashy site and dig up the sounds you just heard with the settings I gave you earlier, do that a while and then jump into all this complicated stuff that is whooping your butt right now but will come easy later. If the site you choose is light on the trash enough, turning fast off, may improve the quality of your tones slightly. I personally haven't found a whole lot of sites like that, myself, and if you use anything but all metal, fast off runs a greater risk of nulling over a good target. Swing slow till you hear a promising tone, then sweep a little faster over it to see if it improves. In fields of plowed soil, I get low and high tones out of a target, I kick away an inch of soil and the tone usually completely becomes low iron (don't dig-nail) or improves to high tone (coin), sweep speed can effect that as well, so don't be afraid to linger over a target and sweep from different angles and speeds. Took you a while to find it, may as well check it out thoroughly before moving on. If the tone washes out to just iron, well that's what it was, move on. Every time I've argued with these machines they proved me stupid. And remember, no matter how much you sweep over an iron nail, it's not going to change into a silver half, I know, I've tried. That is exactly why it is important for you to know what good targets sound like. Once you find them over and over your confidence in the machine goes up and you'll stop second guessing it and will finally be able to hear what it's saying because you are now listening and believing what you are hearing. All this, or you may have a bad coil.
Get a smaller 8 inch coil, solve all at once, for sure. Ease of learning, lighter to swing, better separation, and knowing if your old one was bad. I did that with my F-75 and even with the smaller, new coil, I had the same problems, now I know it's the machine.