My '12 eco did this. It did it one time at like 30k miles, Then again around 50K. Then from about 80-100k it started doing it once a week. Towards the end, every time I started it, it rattled and it was getting louder. The truck was perfect other than that. It was paid off, and I wanted to drive it forever. When I found out it was going to cost around $3,500 to fix it, I traded it in for Raptor. I ordered a 17 and totally got ****** around by the dealer. Ended up getting the first order from them when it opened up I believe in May of '17 for an '18. I received it August of 17. I have 25k on it right now. This morning it is a little cold, so I started the truck as I was walking up, and low and behold, the dreaded rattle... I'm pretty pissed that 6 model years later, I am dealt the same fate. This is crap...
No motor mods or tunes other than a pedal commander...