I am assuming that hydraulic lifter and valve noise are like click-click sounds. I am used to it on my early 911 Porsche(however, the lifters are not hydraulic). This is definite louder, one can hear it across a room. Yes, the oil was low at that time. Wife reports that she didn't hear it this morning after oil was added the day prior. I will continue to monitor. Car is due for an oil change. Will do it this weekend.

John