Heard what I would call in older days, "lifters rattling/ clinging". It was worn camshaft. They couldn't hear it so it got louder and louder. Finally after calling Cadillac, I got it fixed after fussing and fussing. I even explained that this was causing tiny chips of worn metal to pass though my engine and was ending up in my oil pan. They think a lady is so dumb. What they didn't know, was I have in the past helped overhaul older cars with my husband to Cadillac. They gave me 1 yr of engine warranty.