UPDATE: Problem SOLVED!
The metallic off flavor kept getting worse and worse and was no longer subtle by any means. I dumped way too many batches........ Looking back, I should have known better. I use floating dip tubes in both my 10g corny keg fermenter and serving kegs. I had bought some cheap "stainless" rings to move the pick up tube just slightly further below the beer's surface. Turns out those little rings weren't quality stainless and eventually the discoloration caught my eye. They were corroded and the smell from them was really strong for such a small item. I spent a lot of money on going full stainless everywhere in my brewery and was defeated by tiny $.10 POS rings. Copper was never the culprit. I should have known better than take a chance on unknown stainless.