Is it just me or does anyone else experience a vibrating or rattling noise when the volume on their 2012 imac is turned up? i was watching a show and i noticed when higher pitched voices spoke i would hear a rattling noise. enough so to catch my attention, when i turned the volume down (it was initially about 3/4 up) it was reduced but it didn't disappear. is this a defect or just typical of these machines. i have image retention, not too noticeable if the background is changed and i was going to deal with it as i don't want to have to migrate anymore, but if this is a problem specific to certain macs then i will get a repair/replacement.