If you don't hear anything, don't give up! It often takes a little while to get things to work.

As the component tolerances vary, it's unlikely that your metal detector is going to be aligned correctly at first. To set up the metal detector, start off with 25 turns of wire on the coil and try it out. If you can't get a low tone on the earpiece by tuning, then remove one turn from the search coil and try again. Keep on trying, removing one turn at a time, until you are able to get a strong tone in the earpiece. Twenty turns of wire worked in the prototype.

Note: With this design, you should never have a coil with less than 19 turns to avoid interference with other radio devices. You should not need to go this far. Check all the component values if you do.