I have a 2014 Grand Cherokee Limited with 9,500 miles purchased March of 2013 and I am also currently having trouble with a rattle/buzzing noise from behind the drivers side of the dash. It sounds like there are a few potential causes of this, but mine only happens at highway speed independent of throttle position, RPM, or engine load. That lead me to think it is something under the hood "blowing" around and rattling. I think it is too loud to be the washer fluid line like previously mentioned (although I will try securing it to find out for sure) and I also tried duct taping some of the plastic trim pieces under the hood near the windshield and that did not help.
I then noticed that the hood hinge is sloppy on the driver side and makes a loud clunking noise when I grab the corner near the windshield and pull up on it. The hinge on the passenger side is tight. I then found a TSB from 2012 for this exact problem posted by Milous (thanks!!) a while back:Knocking sound from instrument panel area near A-pillar (2011-2012MY)
I am going to set up an appointment this week to have this taken care of. It is a bit frustrating that known issues from previous years were not resolved on the 2014. I also had leaking power steering fluid lines at the cooler which was a known issue on previous model years. Maybe since mine was one of the first runs of the 2014 model, I got left-over parts from previous years...? Good news is my dealer is awesome and getting things fixed has been fairly painless.