The Intex Cache Pro is what you're looking for. The newest model is auto tuning so it remains quiet until it nears non-ferrous. I have the older version which has manual tuning. You can set it to where it barely cackles. Then it'll go quiet when it hits ferrous and squeals when it hits non-ferrous. I can't tell you how many round, rusty steel items it has saved me from digging. Here's a link to the new model.
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in Metal Detector Advice & Comparisons
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The Intex Cache Pro is what you're looking for. The newest model is auto tuning so it remains quiet until it nears non-ferrous. I have the older version which has manual tuning. You can set it to where it barely cackles. Then it'll go quiet when it hits ferrous and squeals when it hits non-ferrous. I can't tell you how many round, rusty steel items it has saved me from digging. Here's a link to the new model.
http://www.intexsystemscorp.com