valves rattling

i have 2000 ford ranger 3.0 , i am noticing valves rattling when climbing hills.
do i need a colder plug. right knw i am running autolite platniums.
i thought maybe the oil was getting to thin, but changed oil . using mobil 1 synthetic 5w 30.any suggestions

Check timing.
Try higher octane fuel.

you should use double platinuim's and are you sure it's a rattle, not a low octane ping?