The noise vibrated like a drill, coming from inside a corner hotel suite on the 32nd floor of the Mandalay Bay Resort and Casino.
At that moment, security guard Jesus Campos was on the floor to check a blocked fire escape door, and he heard that strange sound. Unbeknownst to him, it wasn't drilling, but the rattle of a gun.
"As I was walking down, I heard rapid fire. And at first I took cover, I felt a burning sensation. I went to go lift my pant leg up and I saw the blood," Campos said on "The Ellen DeGeneres Show" in an episode airing Wednesday.
He said he radioed in that shots were fired, but didn't immediately say he was hit. Instead, he pulled out his cellphone, he said, "just to clear that radio traffic" because he knew something serious was happening.
Campos' harrowing account is the first time he has shared his story publicly since getting struck by gunman Stephen Paddock amid the Oct. 1 massacre on the Las Vegas Strip.
After Campos arrived on the 32nd floor at 9:59 p.m., Paddock opened fire from inside his suite into a hallway of the Mandalay Bay just before 10:05 p.m., police said in their latest timeline of events. Paddock then shifted his attention onto a crowd of thousands across the street at an outdoor country music festival.
He fired down through smashed windows, killing 58 people and wounding hundreds of others before police say he turned his weapons on himself.
Investigators say Paddock, a known high-stakes gambler, left no clear motive for the rampage.
Campos in a pre-taped interview with DeGeneres made available online was joined by hotel maintenance worker Stephen Schuck. Schuck had come across Campos, who was lying in the hallway after he had been shot, and credits the security guard with warning him to "take cover" from the bullets, which saved his life.
"Within milliseconds if he didn't say that, I would have got hit," Schuck told DeGeneres.
DeGeneres thanked Campos for his heroism and for alerting the hotel to Paddock even after he was shot.
Campos is walking with the help of a cane, and said his goal is to heal physically and emotionally.
"I'm doing better each day," he added. "Slowly, but surely."