Hmmmm...There are so FEW ingredients in it, I can't imagine what could have caused a metallic taste. Did you taste the mango by itself before you processed it all together? Maybe they weren't ripe enough? I know that when I've tried ones even slightly under ripe, they have an "off" taste to me; not necessarily "metallic", but definitely...well...ICK. I used key lime juice in mine for the citrus, so it's a little sweeter and I always use glass bowls to do heat my curd/creams. Are you using a metal bowl? Maybe it's something that's worn down from the finish if it's an older bowl. I can't think of anything else that would cause that.
I'd be interested in others have opinions, though.