What causes a metallic taste?

And will a bit of aging remove it? I've noticed it occasionally on completely disparate brands of cigars. It's most noticeable if I inhale a bit through the nose as I'm taking a draw and I can really pick it up through the nose off of the lit end while drawing. Just inhaling the aroma of the lit end however does not produce it. It then leaves a metallic taste on the palate but it's mostly in the nose. The taste is still there if I don't inhale through the nose, but it isn't as pronounced. It taste like an old piece of (stainless?) steel that has been exposed to the atmosphere for a while, a "high-pitched" flavour/smell up there with the spice.
Any ideas on what it's from and what can be done about it? Is it permanent, or will resting remove it?