Loose Hinge for 2013 Retina

I would first check the hinges mounted on the base first here. Follow this IFIXIT guide: MacBook Pro 13" Retina Display Early 2013 Display Assembly Replacement. Basically we want to follow it down to Step 9 so the power is removed from the system, then drop down to Step 15 uncovering the hinge screws. Check to see if they are tight. If not you may want to put a small dab of fingernail polish just under the head of the screw to help preventing it from loosening up again - a little goes a long way here!

If you find that didn't do it. You'll need to follow the guide Lapo pointed you to after following this guide removing the display assembly.

Make sure you only go up to Step 4 any further you risk damaging the display! Again use some fingernail polish to help lock down the screws. In some cases the hinge clutch gets worn. You may need to replace them.