Tony, I think he was asking what sort of AUDIBLE volume (how loud) an injector would make, vs. a lifter, etc. Looking for a way to narrow it down based solely on volume, at the moment, it would seem.
if it is a rather succinct, and loud metal on metal TAPPING (loud), then I'd look at bearings, or the like. (You mentioned low oil)
There is a link to an audio clip with a failed (spun) bearing, posted recently. I know Tony2005 can find it. He seems to copy and paste every link ever posted on CEG. He's QUICK.