Either you are just passing over a piece of trash, or you have the sensitivity set to max. Set it at a max of 6 bars. Trash will usually give you a one way signal (iron targets). I don't know if you are new to the hobby or not, but will answer as though you are. One of the key things to listen for when you get a good tone, is to listen/look for a solid regular response. The TID notches are also very helpful in this regard. A target like a coin, because of its regular shape, will give you both a solid tone and usually solid notch reply. They can bounce around a little bit depending on the orientation of the coin in the ground, adjacent targets eta, but in the main, will lock onto a single notch.
Mick Evans.