Hello sbausman,
I had that same trouble some 7 months after my RAI and scared the very soul out of me. My endo explained it was normal and it was caused by the inflamation of the parotid gland, which is also the reason why it's worse when you work out... when exerting effort we salivate more. The metal taste came with jaw stiffness, visible inflamation and quite a bit of discomfort but it doesn't feel the same for everyone. I found it helped to massage my cheeks in front of the ear and the jaw line, pressing inward, towards my mouth... and eating lemons. However, I am not a doctor so I do think it best to check with your endo.
Hoping you'll feel better soon, Shy.