I bought a Garrett Ace 250 a week ago and have started using it only at playgrounds in the sand as the ground is still somewhat frozen. I find it very frustrating in that I have set the detector to Coin and yet it beeps when I scan over bottle caps and cans. I notice that it doesn't beep when I'm over iron, the target ID simply moves over to the iron scale and that's it. I was under the impression it would beep in the coin mode only when coins are discovered. I know I'm new at this, what am I doing wrong? Help.