Some people swear that you can tell what is under a PI coil by the sound it makes.
I have never been able to do this, which means nothing in itself, but I have my doubts whether that line of thinking is wise.
I think not digging any signal based on a PI's sound is a mistake.
If you have an aversion to digging nails or other junk, you should never have bought a Sand Shark. It is a straight beep and dig detector, like any other PI.
If you want to avoid nails, both the Excal and the CZ-21 are excellent beach detectors that offer some discrimination. Granted they are more expensive, but the cost unavoidable.
Any time you refuse to dig a signal the Sand Shark is giving you, you run the risk of not digging a good target, and it will happen eventually.
One day you won't dig a double beep thinking its a nail and it will turn out to be a gold broach with ruby inlays.
That will kick all your previous assumptions about sounds to the ground.
I cannot advise you strongly enough to forget trying to figure out the tones.
If you get a signal, dig it regardless. Thats the best advice I can give you.