tapping in engine after changing my alternator

my dodge ram has 56,000 miles on it. it was running fine and one day the alternator just went so i changed it, well that one was bad too only lasted 3 weeks and the truck just shut down getting off a exit. i pulled the alternator and had it checked and it failed so they gave me a new one i had it checked first this time and it passed. so i went to my truck where it broke down and put the new alternator on. and when i started the truck it sounds like it is tapping in the engine and after 2 or 3 min. of the truck running the oil pressure dropped to 0 and truck shuts down. i always change my oil every 4 to 6 thousand miles, what could have caused all of this just from the alternator going bad.