What Causes Hard Knot On Testicle Which Causes Intermittent Pain?

I have a hard tiny knot in a tube on my testicle. It has been there for around twenty five years. If I wear tight waisted pants or a tight belt, it hurts for days. It is usually not painful? I can roll it between fingers; it is not on testicle, but seems to be inside some sort of tube. As I said, it has been there for many years. I do have epidymitis at least once each year.
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