What to do about the moaning loudly while my dad is sleeping?

I love my dad very much. He just turned 98. He’s bedridden and on hospice. I’ve been his only caretaker since 2016. I’m sorry to say this and I know it’s not his fault but his groaning loudly has become very disturbing to me. He’s not in pain. It probably from nightmares. I have woken him up but it startles him and I’m afraid to keep doing it cause he has irregular heartbeat. I sit with him all the time, watch tv in his room but when he starts the noise I have to leave. I’m very tired.