Read a lot of this, helpful tip I got a while back - vibration in the steering when braking is front rotors, vibration in the seat is rear rotors. I don't like to tell people how to do their jobs, but sometimes service techs need to be told. I took a previous car in for a similar issue, and the printout when I picked the car up said "ran
scanner, no codes". LOL obviously had to take it back so they could ACTUALLY check the brakes.
That's a shame and I disagree with an earlier poster saying this is not lemon material (I've been through the process). In most states, if the issue significantly affects drive-ability or safety, you fall under the coverage. Sounds like you do.
The metallic 'slamming noise' is most concerning as it sounds like a hardware problem in the rear diff...not as potentially benign as some bucking or hesitation (software issue, misfire, etc.). Do you find the car exhibits the same issues in all drive modes? Unfortunately yours isn't an AWD, may be able to get a few clues based on drive mode/power transfer % to front/rear. Does anything in particular generate the noise? Partial throttle, manual shifting, does it do it when it right when shipping from R to D, etc?