Muttering - What should I do?

My boyfriend is OCPD or is diagnosed as such. I myself am schitzotypal, so I can handle quirks. I have not been able to find anything on the internet about his most prominant "symptom" however.
He mutters under his breath and sometimes it is terrible things like "i am going to kill you" or "kill yourself" or "stab you" or "kill them all" or it can be totally nonsensical like "chicken pants" or "baseboard" or some other seemingly random phrase. Sometimes when he says something awful like "I am going to kill you", I have responded and he has been embarassed and apologetic so I know this can be troublesome to him as well.

I guess I want to know what is going on inside his head - it often sounds like a continuous dialogue that I am only privy to parts of.
He says he doesn't know what he is saying, but sometimes he hears himself.
In public he mutters quietly, but at home and around me he is more comfortable I think, and so it is more prominent.

He is a really nice guy, with social anxiety, I know when he is stressed out or tired it happens more, so he isn't able to control it as much.

I would love to hear from someone who mutters to tell me what is going on from his point of view.

Any suggestions or help is appreciated.

Hi,

Could it be Tourette's syndrome that he has? It's certainly puzzling anyway. Did you ask him if he ever means the things that he says?

Kevin.

Hi thanks for answering.
I too have thought about Tourette's. Maybe he is mis-diagnosed? Or has more than one thing going on.
He says he doesn't even know what he is saying, and he don't know where it comes from.
He has said that he "isn't talking to (me)" and that none of it is directed at me.
Sometimes it does seem appropriate commentary, but most of the time it is really out of place.

Yes! My fiance mutter's under his breath when he is very stressed or angry at me. I hear him and ask him what he is muttering and he denies doing it. He also talks in his sleep and has an angry dialogue with someone. I have not seen this as a symptom of OCPD but now I wonder?