had a slight engine knock under accelleration that increased / decreased in speed along with acceleration / deceleration. had plenty of good oil in it. added 2 quarts and in the process realized my oil cap was almost completely off. continued driving with lighter intermittent knocking til i got home. not sure if fixed. last time this happened i realized i was low on oil as my drain plug had been leaking so when i changed the oil it stopped the knock. this was less than a month ago. any ideas?