That usually happens when you inadvertantly push the PP button while over an adjacent metal target (or too close to the original large target). When that happens, just go out to the opposite side of the target, press your PP and then pass over the target again. Also, when this happens, it confirms there is a potentially masking adjacent target which when you know that, it can help in your final decision to dig or not. Another possibility is that you are just hearing a false tone off of a ferrous target in which case the good tone isn't over the actual object and you can confirm that by doing what I previously mentioned from multiple directions.