What does it mean when the engine knocks

Most likely it's tappet or lifter that are not "pumped up" yet

They may be dirty

Since you said it does get better as you drive, I'm leaning towards the tappets, It is possible that other components may be worn also

This answer, in the link below, is the same advice to you

https://www.2carpros.com/questions/2000-ford-ka-tapping-from-engine

Keeps my pecking to a minimum too

See this too!

http://www.youtube.com/results?search_query=marvel+mystery+oil&oq=marvel+myster&aq=0&aqi=g4&aql=&gs_sm=c&gs_upl=1255l8663l0l11311l13l13l0l3l3l1l446l3848l3-7.3l10l0

If you return tomorrow, in 2 weeks, a month, or even a year...TO THIS SAME POST....I will be notified, and you can post your results!

The Medic