Johnny Klebitz was the president of The Lost MC, and the playable protagonist of The Lost and Damned. He was killed by Trevor Phillips during the campaign in GTA V.
The myth is, that after the mission Friends Reunited, the ghost of Johnny Klebitz can be heard (or rarely even seen) in Stab City, where The Lost MC are located. Players have reported hearing Johnny's ghost screaming things like I'll get you for this, motherfucker! or I will shit all over your company! Some players reported seeing, or hearing him where he was killed. A newspaper clipping featuring Klebitz can be found at the Blaine County Motel, which also happens to be a hotspot for ghost sightings.
No videos, or concrete evidence has ever emerged, and no files exist in the game that are related to the supposed entity, and Lost Bikers are rarely seen after his death, except for the random events and missions, making it even more absurd.
The noises that are sometimes heard are completely false and faked, as his dialogue is only attached to his character in the files, making this even more absurd than before.