Check all exhaust connections. I was getting a metallic rattle under some load at low RPM in 1st and 2nd on a cold engine. Thought it was the DMF, turned out to be the clamp from the DP resonator to the catback.
That would line up with the issue when I was having it. I spent an hour under my car, and didn't find my exhaust touching anything, anywhere. So the clamp from the downpipe to the catback being a little too loose and rattling makes sense.