What remedy can I try for constant moaning from my 90-yr-old mother suffering from onset dementia? She is not in pain and won't talk.

Get a noise cancelling headset for yourself.
Play music for her.

This can be a very common habitual sort of circular action. I thank Lea for the link below. I agree also with JoAnn that you should discuss with Mom's doc who knows her best. Some have good results with the more mild and low dose anti depressants for these sorts of behaviors.

Good luck. Please update us if you find something that works.

Here is a link to an article about moaning with dementia and strategies to deal with each possibility:

https://safesmartseniors.com/why-do-dementia-patients-moan/

This article doesn't address Sundowning which happens in the afternoons and brings anxiety and agitation along with it. My mother had good results with Ativan .5 mgs to help her relax a bit and calm down.

Good luck.

Talk to her doctor. Maybe its anxiety and a medication would help.