That is interesting info. I read a bullitin that said if the lifters were not staying pumped up and/or the rocker shafts were worn then the oil pressure relief valve was probably stuck open. I'd never heard of this valve. I replaced the lifters on my 97' Rodeo only to hear the same lifter noise as before. I also noticed when cleaning the rockers and shafts that the shafts were worn. So I guess I'll be pulling those valve covers back off and checking out that valve.
My motor has 111K miles on it. Do you think plugging the hole would be risking causing another problem?