Possible that the oil drained completely out of the filter assembly, as that is not usually the lowest part of the oil system on these cars. That momentary noise is from oil being pumped around until it builds enough pressure / oil film to do its job.
Not unusual - pretty much every car will experience this no oil / low oil pressure / oil film on a cold startup. These particular cars are known to run pretty low oil pressures to start with (recommended low viscosity oils don't help either) - so you might be able to pick up the knocking noise until the pressure builds.
Variable valve timing setups are also known to cause these knocking noises on cold starts - as many times they are also driving by oil pressure.
Sounds like you are one of those owners that are blessed/cursed, depending on the situation, with above average hearing.
All in all - don't think this is an issue. As long as you stay ontop of oil changes and the car only makes this knocking noise for a few seconds, I think you are likely not going to experience any issues down the road.
That said, probably couldn't hurt to have some paperwork started on this - as you are well inside the powertrain warranty on the car - better to view this as a future safe-gauge - just incase this turns into an "issue" down the road.