How do you know? you take off valve cover, rotate engine untill cam tming marks like up on there marks then read crankshaft marks , usually your going to find one mark a little off, but i change them at 4-6 degrees off, 2-3 its not going to matter but ive seen them as high as 8-9 degrees off reading at the crankshaft, dont wait above 120,000,do as everybody advises inspect now for source of the rattle, ive removed drivers valve cover when there was no complaint and to my horror found the plastic guide already broken and dropped but didnt catch the chain to do damage, you can attach new chain to old and rotate after installing if you want to just install chain and upper guides, you can but thats another subject.