2018 Mustang Gt cold start rattle

I have a video, not sure if I can post it here, anyway here is the issue. Car has a brand new replacement engine on a 2018 mustang gt with less than 800 miles on the engine. It has been in a ford shop for 3 weeks now and they cannot locate the issue. They thought it was the timing chain tensioner on the drivers side , which they replaced and still no go. It only does it on a cold startup and if the car sits for more than 6 hours otherwise when the engine is so called warm it makes no noise on restart. They supposedly checked the VCT solenoid and the Cam Phaser on the drivers side of the engine, now they think it may be the cat on the drivers side, which makes no sense to me.