If there is no sign of oil leakage and you are sure it was full 3 weeks ago then you might have a blown head gasket and the oil is slowly being burned up. The knocking might be pinging because of a fouled cylinder. The capacity, as you know, is 6 quarts so you still had 3 quarts in the system. The timing chain tensioners are dependant on oil pressure as well, that is why we get that clank clank at startup, and that could have been what you heard as well when the preasure was low during the right hand sweeper.
Just trying to be optimistic.
Good Luck
