Same thing happening with my car, My car has very very little rattle on start up now. Two causes of concern and you should defintely get it check out ASAP!
1. - KIA Corporate Regional Parts & Services Manager stated that there are known bad oil filters that cause improper pressure for the engine resulting in longer period for the oil to reach the engine on start up. (This wasn't my case)
2. - (My Case) I had $3,200.00 work done to my engine components because of a CVVT issue. They replaced both cam shafts, electric timing chain and etc....... This fixed my issue. Car rides like it did when I first got the car.
I have a rattle at start up, now a rattle in the dash (heard when going over rough roads/bumps), and on occasion I hear a creaking noise at low speed. I've got to call and schedule to get an appointment to replace the outside trim on the rear passenger window, so I'm going to have them check these items out as well. If this cant be fixed, then I'll get my Audi sooner than I wanted