George Zimmerman Traffic Stop -- Cop Under Investigation for Sneaky iPhone Pic

It's the most recognizable iPhone sound on the planet -- the camera shutter -- and now it's biting one Texas cop in the ass ... after he apparently snapped a sneaky iPhone photo during the George Zimmerman traffic stop this weekend.

Law enforcement sources tell TMZ, the officer who pulled Zimmerman over for speeding on Sunday is currently under investigation after dash cam footage revealed he took an iPhone picture during the stop ... at least that's the way it sounds.

You can plainly hear the telltale sound in the video -- followed by the "unlock" sound and typing iPhone keys. You'll hear it 29 seconds into the video.

It's unclear what photo the officer snapped -- or if he sent it to anyone -- but if he took a pic of Zimmerman's driver's license for personal use ... he could be in trouble.