It could be yeast settling out. I've had a slight metallic taste early in my kegs that cleared after a few pints and I'm pretty sure it was from the yeast that was still settling in the kegs. This was even after pouring off the first pint from the keg.
Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive. It would be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience. - C. S. Lewis, English essayist & juvenile novelist (1898 - 1963)