Iron can sound a little fuzzy compared to gold or lead, aluminium will sound very bright. Some of the microphone preamps I've used over the years for recording music have different types of transformers on the outputs, some nickel wound, some silver wound, some with an iron core and copper wound. Each of these types of preamp transformers will sound very different depending on what metal is used in them and each inherent difference will suit particular instruments or a particular type of "sound" such as rock music. For instance preamps with iron transformers sound fantastic on heavy electric guitars giving them a bit of a fuzzy sound compared to say a very bright silver wound transformer.
http://www.adesignsaudio.com/500-series-preamps
My point being that each type of metal and configuration has it's own sound type. That being said, pretending you can know the difference between gold and trash with non discrimination detectors is going to lose you alot of gold as there are so many variables. Plenty of experienced detectorists say, "hey look at this 2oz nugget, sounded like trash but it wasn't".
In my opinion and experience iron is the easiest signal to pick due to the fuzz sound in the signal but plenty of nice nugs are encased in ironstone so.... dig everything with PI's and ZVT.
