Yep, sounds just like rusty iron nails or what not. You're a fast learner. A lot of the iron will chirp CO 47 or 48, but you'll notice the FE is usually 20 or higher. When you switch to quick mask you usually get a solid FE 35 with a CO of 47,48,49 or 50. Also, if you X the iron spot, it may chirp from one direction, but not the other. This is a pretty good indicator of iron.