NFC is off and there are no RF chips near by. I have had it make the same sound during calls. not sure of the circumstances with those since i don't really use the calling part of my phone very often.
i can replicate it by pressing the screen lock on then off and putting it down. but if i just pick it up and put it down it will not make the sound
this was an issue with my S3 but it came over with the new phone and google porting over all your settings to be helpful.
and to make things worse the sound was changed by a soundboard i downloaded a long time ago which i don't remember which one so i cant even tell you the sound.
any help would be greatly appreciated.
