Grinding, " diesel " noise under valve covers

Yeah I was thinking maybe the R and L got swapped? But looking at them that would be damned near impossible.
Well, they were New MP. HD units. I used a book to put em on.... now when you say " swapped" do you mean bank to bank? Or dig up some 318 stockers, or did you mean I got the R @ L swapped during install? At this point, I ain't ruling nothing out. And this " sound" I did notice awhile back. I just want to find out what it is, and rule out Anything that could cause CatEngFail