2000 ford ranger 6 cylinder engine is making a taping noise?

I left it set more a couple months and now it is making a taping noise when I start it. My friend had a stethoscope with a rod on it… He held it to the valve covers and block to listen. I heard nothing on valve cover and only heard it slightly on the block, but it was really loud on the alternator… Does that mean it is my alternator? How accurate is a stethescope? Also, the battery was dead and we jumped it… Not sure if that matters… Any ideas?

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