Both my sons have done that from birth, in fact it is what alerted me often to the fact they were actually falling asleep.
It's not a frog croak just a little murmur, as one might say 'Mmmm' in agreement with something.
I'm not aware that I do it but I know my son (40) still does.
Is it normal to talk to yourself?
Using equity release to buy partner out of home


