Based on all the info you gave--you MOST likely have 1 of 2 possible conditions:
1. Chronic, Intermittent (non-infectious) Sinusitis: [THIS is MUCH more likely]
This can be caused by seasonal (or episodic) allergies, which cause increased sinus fluid formation (this can taste 'chemical-like; metallic; bitter; salty...'; I've heard MANY descriptions used), which then drains down the back of the throat--and your taste/smell receptors detect it.
2. Injury to your Olfactory (smell) or Taste Nerves [called Cranial nerves]:
(this is LESS likely than #1, but far more serious)
There is the POSSIBILITY that a lesion in your brain (eg. cyst; fluid; aneurysm; blood vessel malformation; or even tumor) could be compressing (intermittently) either the taste or smell nerves, thus causing the symptoms you describe. Since sx also occurred last year, I would talk to your MD about getting a CT or MRI Scan of your Brain done, to RULE OUT the above pathologies [again--the odds are LOW, BUT it is great to be conservative, and to have piece of mind, I think you'd agree].
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Best Wishes for Your Health,
--Dr. Steve, (Your Online Family Doctor)