is that because the lifters should normally retain a certain amount of oil when the engine is off? if mine are not doing that can they be cleaned / relubricated / or something or is replacement a given? How big of a job is this?

Also, after that incident, I started listening really closely to my car on a cold start and on a cold start you can hear the faintest of a similar knocking sound. But this is super faint, you'd have to open the hood and really get your ear down close to the engine to hear it.