17 month old moaning in her sleep

[Flossy1980]

Hi, my 17 month old has never been a good sleeper always waking lots during the night. She has finally settled down and sleeps though most nights. The only problem is she moans in her sleep. When I check on her she is asleep just making a constant moaning noise. If you change the position she's sleeping in it will stop but will start again in 30-60 mins and happens from around 11-4am. I've read about nightmares and night terrors but this doesn't seem to fit as she isn't upset or waking.
I also doesn't happen every night we can get the odd night where she sleeps through without any noise. I've monitored naps etc but there isn't any pattern I'm totally stumped. I've mentioned to the HV but she'd never heard of it before! Any help as to what I can do to stop it as I'm exhausted from getting up every hour!
Thanks