My 2014 5-speed hatch SE runs very smoothly at any speed above 1000 rpm, but has a significant and annoying vibration at idle speed--worse if the A/C is on.  It makes the whole center radio cluster shake!  The previous owner told me he installed an aftermarket "stiffer" rear engine mount (he didn't mention the other mounts).  Would appreciate opinions:  should I install a full set of OEM mounts or is that a common and unchangeable issue?  (The car has 94,000 miles).  I'm not interested in 'improving' the car like performance-oriented owners--I'm an older guy who likes smoothness and quiet!  Thanks for suggestions. 

The upgraded mount is stiffer to decrease engine movement but can cause more vibration. An OEM mount should be softer and dampen the vibrations better. The MK3 mounts are common for deteriorating. I would first try the rear mount underneath and if that doesn't solve it then the driver side one next then the harder to get to passenger side.