I picked up a used X-Terra 70 the other day and I have a question about the tones. When ever I turn it on, it has a constant tone coming from it even though it's not over any type of metal. The tone will change pitch when over something, but it will still be there when you move it away. The only time this constant tone goes away is when I select pinpoint. Also, lowering the volume doesn't help either. I haven't been able to find out anything about this to know if it's a problem or not or if I purchased a malfunctioning detector. Any help with this would be appreciated. It also has 2 coils, a round and DD, and does it with both. Thanks for the help.