Lost 2 quarts of oil in 50 miles?
 

Drove interstate - 50 miles, off ramp heard loud knocking/clicking at stop light. RPM up noise went away, Stopped 2 quarts low added, same results while driving. No leaks under or around engine. Drove back home, inspected in/out of turbo no oil, added another 2 quarts. Low idle pressure +/-30#, 900 rpm's +/- no noise. Seems at 40# noise goes away.
Where did the oil go, no black smoke, water or oil at exhaust pipe, and what is the noise?