Husband has started nonsensical babbling. He makes up a phrase of babble and repeats it ad nauseam for hours, like "lee-la-loo-la". Advice?

I would talk to his DR. This maybe anxiety and there are meds for it. Since he has a hearing problem maybe he doesn't realize he is doing it or is too loud. It also may be a comfort thing for him.

Ear plugs.
(no I'm not being sarcastic, it has often been helpful for me over the years)

Earbuds for your iPhone when necessary to be in the same room. Play music or listen to those background "white noise", "music in another room", etc.

The simplest solution would be to ask your husband to stop the babbling, since he seems willing to do so. He may not even realize he's doing it since he has a hearing deficit and may not hear himself making those sounds; perhaps he thinks he's humming to himself but you can hear it, in reality!

Good luck!