NJMIKE, the threshold will never stay steady, as you describe it. The silence that you are hearing is the iron masking. Set the threshold at just above the point you can barley hear it, and leave it be until you gain more experience. You need to plant yourself a metal garden of gold, silver, aluminum, and any other metals you can think of, and get yourself used to the different tones associated with each. These are the tones you are looking to hear. A lot of the other noise is just that, noise, frequency changes etc. For me aluminum still sounds like gold.
Follow the recommended settings in your owners manual, and get out there and practice. It will take some time, you will sometimes get a little frustrated with it, but just keep at it and you will get the hang of it.
Tom