this sounds like one of two things.Either its a worn lifter/cam rocker or a rod bearing failure.It really depends on the sound it makes.The rod is lower inside the engine and makes more of a thud to the click.The lifter/cam rocker is high in the engine and make a higher clicking sound due to less mass.If I had to make a bet I would guess valve train noise.If its suspected as valve train pull valve cover off and let engine idle use a soft hammer handle(ie wood rubber)and push down on the lifter pivot this will tell you witch one it is and if it is the lifter itself.The sound should change on the bad one.Now if it is cam or rocker pry up on the same one(pivot may have to use a screw driver) and the noise should go away. Good luck.
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