befor u do the oil get a really long screwdriver n put driver end on the rocker cover
put your ear on the handle end and listen .
try it on top of each rocker.
do the same on the injectors.
you should be able to tell which it is injectors or some thing in the valvetrain, and go from there
could be nothing but hard to say as only u can hear how loud the tap is .
but some tap from the injectors is normal ,
good luck