My 2014 started chirping before 500mi, then I stepped on the gas. Hasn't returned, but I'm still under 1000mi, so no guarantees on that as a fix. Mileage fell from 31.5 to 28.5 as well :shrug: I feel like the car doesn't behave well when it's sipping gas.
My car "idles" at ~600rpm all the time -- when it's in gear (A/T). A/C isn't a bad guess as a load source -- maybe the alternator (is it dark when this happens)? Pretty light as a load, but maybe you catch a transient? Like I said, though, when sipping gas, the car doesn't behave nicely. It feels like it's constantly changing it's mind as to how it wants to run. I'm guessing the engineers didn't spend a lot of time working on the helium-footed 0-12mph range
I find this car to be fine wrt mileage. I get better mileage than my wife's Elantra, and better mileage than the Saturn it replaced.
I don't have any clacks, but the car REALLY doesn't like turning in the first quarter mile, and it hums/groans/softly-wails when stopped (and in gear). If it ain't one thing it's another. :shrug: