I've done a few MRI scans around the head (both 3T and 1.5T machines) and I tasted this metallic taste every time, despite staying still as much as possible (I'm not so bad at it).
Interesting - if it happened during the scan it would be due to the gradient coils, not the main solenoid field. The gradient coils are much weaker but turn on and off during the scan (the main solenoidal field is static).
