Oh, Rosie, you have my sympathy. I heard so much of that in the nursing home where my parents were, and my dad had days like that. When people have dementia, it's so frustrating, because you want to make them comfortable if they are having pain. But they can't communicate, so you don't know what it is that they are moaning about.
It's possible this is just reflexive, which doesn't help you, but if you could know he's not moaning out of pain, it would help.
I'd have his medications checked to make sure he's on the right ones, and then make sure the doctor knows what is going on. Maybe there is medical help for him.
Meanwhile, blessings to you. I've been there.
First, I believe you need to to determine what may be behind his moaning and groaning. Often, a loved one with Alzheimer's cannot describe if they are hurting or ill. Therefore, I'd strongly suggest that you set up an appointment for your father to see your father's doctor to determine if there are any underlying causes for these moans and groans.
Take care and keep us posted!