Thankfully no one has been dumb enough to argue the CDC whisper said it was about people turning without bites. Rick didn't have any idea of this until Carl just shot Shane, or else he would've put one in Shane's dome after killing him. That entire distraction is dead now, thankfully.
See, I took that a completely different way. While tore up over his death, he took this as a way to test the theory that he hadn't had the luxury of testing. Unfortunately, Carl had to come running out to "help", but I did like the little homage to the comics there. But it seemed like he was waiting around to see if Shane really was going to turn, and was ready to put him down for good if so until Carl showed up. The hanging would have been the test but maybe he decided not everyone was ready to know this yet, as it would kind of suck the hope out of you. But regards the hanging I think they were just going through the different ways of killing him.I do believe everyone is infected, and I think the writers have been making that very clear for the last 2 or 3 episodes. In light of that, I think it's pretty safe to say that was what the CDC guy whispered to Rick and why he committed suicide.