What could this tapping noise be? Lifter, or something else?

i was going to say a rod bearing but usually a rod bearing once it spins it won,t stop knocking. i would have to say you have a lifter sticking. as for the oil pressure it could be the oil sending unit acting up. usually about oil pressure you will have more pressure when the oil is cold then it will drop off once it gets hot. i would say change the oil to 10w30 and change the filter. it would,nt hurt to throw some good conditioner in the oil either. get something like BG oil conditioner. most garages sells BG.