Drove it - reached destination and started knocking - NO oil on dipstick. Put in 5 quarts of oil - ran fine until reached next destination - started knocking again - oil was present this time - drove one more time and again started knocking at destination and oil present on dipstick. Knocking does not sound while driving - just when slowing down. No warning lights came on at all. No sign of leaks or burnt oil on tail pipe - any idea what needs fixing?