I was just running into this today for my self - and ended up doing what you all recommended: move around the target, vary coil speed/angle, check it in all metal/pp, back into discriminate, then dig it up. So far, they have all been junk.
But, I also heard a sound that was different than anything else (which was ALL pull tabs), and worked it the same way I had been working the nulls. It sounded so much like aluminum it was sickening - but there was a difference too. I want to say it was cleaner somehow, but just a little bit. It turned out to be a nickel. It was the only good target of the day, the rest was working with nulls and pull tabs; but I learned; and hey, I picked out a nickel.
It's not much, but to me it was important that I heard the difference.
HH
