MN does have a state warranty law for "X" number of days "X" number of miles, I do not recall what it is however.
May I ask what dealer?
What city are you in?
If you are in the **** Rapids(surrounding) area I would not mind taking a listen at a cold start to confirm.
The description sounds like piston slap, but internet descriptions can be very misleading.
EDIT here it is:
Minnesota (MN) Used Car Warranty Law - AUTOPEDIA® - The AUTOmotive encycloPEDIA
