Husband talks to himself?

My husband and i have been together for 6 years, its a little embarring but there is still one thing i do not know about him after all this time. We are a normal couple, generally our relationship is great, we have a son. My husband is extremely smart. He can relay a novel if you asked. very smart. But one thing ive noticed is that he talks to himself. He does it when no one is around or he thinks no one is around or when he thinks i cant hear him. He whispers really quietly. He has pretty much conversations with himself. I've tried many ways of approaching this subject so he will tell me why he talks to himself. But he just says its nothing. An example i was sleeping and i heard him getting ready for work in the morning and i heard him talking to himself. If i pretend like i think he is talking to me and i say "what?" or "i didnt hear you" he just says its nothing. On one instance i have asked him if he's talking to himself, and he says it nothing he's just thinking. Or if i ask him why do you talk to yourself, he replys with its nothing. Now i dont mind that he talks to himself, but it just concerns me, that this is one thing he will not share with me. Also it somewhat scares me for the fact that sometimes he says strange things. We have cell phones that when the key pads are not locked, we accidentally call each other. When i am at work and my phone is off, and i turn it on to check my voicemail, i will have 15 minutes of voicemails from just him accidentally calling me from the inside of his pocket, or the sound of the tv, or driving, ect. Seldom, i do get a voicemail where he is talking to himself. This particular voice mail was when our son was 2 months old and it really scared me. He was very hard to hear, as he whispers when he talks to himself, but the gist of the conversation was that he was saying "wait for them to go to sleep and then boom and theyre all gone" That sounds like a crazy mans talk. He doesnt at all have the behavior of a crazy person. I was so worried about this voicemail, also because of the strange voice he spoke with, that i confronted him, and he just pushed it off. I started out saying i heard him talking to himself on a voicemail, he just pushed it off to be nothing, then i hinted at it, and he said it was nothing. then when i repeated what he said in the voicemail, he said he was watching a movie and .. then he just kind of muttered excuses that i couldnt follow. So, does anyone have any idea about this? Is this normal? is there a mental illness behind this? This just worries me because we do have a newborn baby, and the fact that he does have some anger issues, and by that i do not at all mean that he is abusive, he just has a temper control problem. Also, why will he not tell me why he talks to himself? 6 years and i have only been able to hear him maybe 3 times, hes so quiet and doesnt want me to know. Also, younger in our lives, before we were married, when he lived with his best friend, his friend said that my husband creeped him out by doing this. Any help, any info thanks.