Knocking and tapping are two different things. Knocking is more of a 'lets take a hammer and hit a metal plate' tapping is 'lets take a screw driver and tap on the same plate' . Seeing how its doing it in the colder months, could be you have a lifter going bad/collapsing. Just taking a while for the oil to come up to temp. Try a thinner oil (5w-20) and see if that helps at all.