I love my 2014 Cadenza SX-L. It's been by FAR the best car I've ever driven, and I wasn't expecting the amounts of compliments I get from Cadillac, Lexus, Infiniti owners! I've definitely strengthened my brand advocacy for Kia/Hyundai. (Previous: 2009 Hyundai Sonata Limited 3.3L VVT V6)
ON THAT NOTE: This **** rattle for a second or two on startup was getting the best of me. It was embarrassing, as you would probably know. Especially when your coworker with a 2011 Ford Fusion starts pointing and laughing. I just turned over to 12k miles last week.
2 weekends ago, I had about 550-mile round trip, driving 70-80mph on interstate. BEFORE I departed, I topped off the tank with 91-octane (already had about half-tank of 87-89, 10% ethanol gas), AND the Kia-recommended Techron treatment. I had already treated the tank with Techron that I purchased from my Kia dealership about 6k miles ago. The mistake that time was that Kia only had the smaller bottle on hand, that "treats up to 13 gallons" or whatever. This time, I went to Mills Fleet Farm and got the larger Techron bottle for about $14 that "treats up to 20 gallons". This was only about $1 more than what Kia charged me for the small bottle, and considering the tank is over 18-gal, it just made more sense.
Anyway, I am on my 3rd fill-up since this trip and Techron/high-octane fuel shock. I use regular gas, usually from the same local station. My Cadenza sounds like it should! Absolutely no rattle like before. It makes me happy hearing the nice whirr from a normal, healthy premium sedan V6 engine.
I am no mechanic by any means, and I have absolutely no idea if this in fact is the exact fix I needed. But it worked. If it's just a crazy coincidence, I'll just roll with it! But I did nothing else that would have gotten rid of the rattle.
Another note: It has since also gotten colder in the mornings here in Fargo, ND - and there is still no sign of any rattle like before, even on 40-degree early mornings.
Hopefully if you attempt something similar, it will work!