Dog Waking Up Each Night Growling and Barking

We have been having some issues with our 8 year old dog waking up and growling/snarling/barking in the middle of the night.

It started a few months ago shortly after we moved house and we had originally put it down to the new environment but she doesn't seem to be stopping. We took her to the vet to get checked out and the vets have no idea what could be bothering her, more details below.

She is an 8 year old lab and we have had her for 3 years, she was a rescue and previously used as a breeding dog in a puppy mill. We have another 2 year old lab and they get on great. Both dogs have slept in the room with us the entire time we have had them.

At first, we thought that maybe the younger lab was disturbing her sleep so we gave them separate beds then we even moved our older dog into the room next to us to give her her own space. She doesn't have separation anxiety and will happily snooze away in different rooms from us so she will fall asleep no problem on her own.

She has never been a snarly/growly/barky dog and the behaviour doesn't seem normal but we are at a loss as the vet doesn't believe that there is anything physically wrong with her.

At first we thought she was growling/snarling/barking in her sleep as she would stop if we called out to her but we have used a nightvision camera to check and she seems to be awake with her head up. She doesn't lunge or show any agreesive behaviour to anything in the room - ie. Us or my other dog, she just looks ahead snarling and barking.

She wakes several times most nights and starts growling, which turns into a snarl then finally a bark - and it sounds quite agressive. She doesn't exhibit this behaviour during the day when napping, only when sleeping at night.

We had thought it was maybe a sleep-startle reaction from our innitial research, but there isn't any noise in the house or areas at night and we have given her her own space so I don't think it is that.

While it is an inconvenience for us as we wake up several times each night, I am more concerned about her health as she is clearly having disturbed sleep and something is bothering her.

She is walked regularly and goes on pack walks with a walker some days also, and we play mental games/search and sniff games etc so she is well tired out each day also.

I am wondering if anyone has experienced this before or has any ideas?

Thanks!