I can not say for sure what is causing the metallic taste, but it is possible that it brain chemicals being released during practice.
Check out http://metatoninresearch.org for some insights: search the word metallic.
If this is it Basically, don't worry abut it. It is a side effect of your practice and actually sought after by meditators.
Curiously, the same effect is being had by people applying electrical current directly to their head to improve brain function.
Interesting piece on Radio Lab about this: http://www.radiolab.org/story/9-volt-nirvana/
be well, Tom
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