Got 880 miles on my new 2013 evo x gsr. Just yesterday I realized a weird vibrating when 2k-3500rpms and only 3rd and 4th gear sometimes 5th. Its extremely unsettling because I just got the damn car. What could be the problem?! Anybody else get this problem before?

how much throttle input when you hear the rattling. are you WOT in low rpms?
just so much you can go off of here.

I have an MR so if i'm in manual mode and forget to downshift that could most likely cause a rattle or vibration.

also could a panel that is just a little loose.

once again. can't stress enough, as much info as possible would make it easier for some of the vets of this site to help you.

get used it plenty of more rattles and weird sounds to come

Check to see if the exhaust is rubbing on the sub frame somewhere. I was experiencing a weird rattling when I got mine too and checked to find that out.

I posted the quick fix to this with a picture in the Rattles thread over in the how to section. It's the spring clips vibrating against the stop. Takes about 5 minutes to fix