Hey guys my names Zack, I am new to this forum. About 8 months ago I bought a used 2012 F150 Ecoboost. I love the truck, its been loads of fun. The original owner did a few mods including a heartthrob exhaust system and a superchip tuner.

I recently started experiencing a loud grinding noise for 3 to 4 seconds when cold starting the truck. I flashed the truck back to stock, and I took it to my dealer (with a recording of the noise), they were unable to replicate the noise so I was told to bring it back when I am able to leave it for a few days.
A few minutes after leaving the dealer, the check engine light came on. I scanned it and it came up as P2096 "Post Catalyst Fuel Trim System Too Lean Bank 1".

The truck is currently spending the night at the dealer with hopes to replicate the grinding noise, but they just called me and let me know the cause of the check engine light is my exhaust system is "too large" and causing the truck to run lean. They told me since it is aftermarket, there is nothing they can do and they dont know what I should do.

My question is, how was I able to go 8 months (and however long the previous owner went) without this issue? Only things that have changed recently are flashing the truck back to stock, and the weather has got much cooler in my area.

Any idea what I should do? The dealer wasnt even able to tell me if this is damaging in any way to the engine.
I am anxious to hear from the dealer about the grinding noise too, sounds a lot like timing chain issues I have seen in a few videos!

Any help is greatly appreciated! Cheers.