Insane Clown Posse Rapper: Someone Jacked My Car Stereo ... IN TEN MINUTES

Violent J -- one half of the rap group Insane Clown Posse -- says he left his car unattended for a grand total of ten minutes yesterday ... and when he came back, his car stereo was GONE.

Sources close to Violent J -- real name Joseph Bruce -- tell us, the rapper and his pal were making a quick pit stop at some store in Detroit when the robber struck.

But according to sources, J blames himself -- because he left the door to his 2010 Suburban unlocked. Probably not wise.

We're told J doesn't plan to file a police report -- he's gonna suck it up and buy a replacement.