The rust from an iron target isn't normlaly enough to cause an issue but what you are seeing is a very strong return from a large iron object that causes your receiver to be swamped by the internal magnetic alignment inside the iron target. In a ferrous target, the magnetic flux lines are developed inside of the iron object setting up a N-S field which collapses and is fed back to the receiver. That collapsing field is sensed by the receiver coil and then amplified in a very high gain circuit. If too large a signal comes back, it will overdrive that first stage causing what is referred to as swamping. Basically, it's like throwing a bucket of water at a shot glass just to have a drink.
Now, if the iron object has seriously degraded and is flaking apart into the surrounding soil, then you have a different effect. Many small magnetic flux alignments in many small iron items can cause eddy currents of magnetic flux which are very unstable. It makes a warble to the receiver and the result can cause some strange audio signals depending on how your detector generates its tones.
There are some patents you can search for and read that will help to explain this. Go to the US Patent and Trademark site and search for metal detector patents. There will be hundreds but a lot of them "borrow" from each other.
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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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