You might try asking your father directly. I wouldn't act alarmed or annoyed, just mildly curious and see what his reply is.

I talk to myself and have done for probably the last 8-10 years, since my child was old enough to leave the house in the evening, and I would be all alone with just the cat. I talked to the cat at first but now I just talk out loud, as if to another person, but really I am just voicing my thoughts aloud. I get tired of the silence in my house and I get lonely sometimes. I don't do it all the time, and it does seem to relieve my stress.

Your dad might welcome talking to you about what he is doing, or maybe not. But you won't know until you make the attempt to talk to him about it.