Cat will not stop meowing

I am a very experienced cat owner and foster. I have had cats for almost 30 years and have fostered over 100 but this is a first.

Over Memorial Day weekend I acquired a very young mother cat and her 3 kittens from a rescue partner I volunteer with. Mama cat was only about 5 months old herself and less than 4 pounds. I have had her and her kittens the whole time, keeping the kittens in a guest room and Mama able to come and go once the kittens were eating solids.

She is not lonely, therefore, on either side of the door. She gets tons of human interaction, all day access to very good quality food (her weight has more than doubled) but she yells nonstop.

She’s not in heat - I know what that looks like. She’s healthy, she loves people, she has no issues with our resident cats, and she will not shut the fuck up. She meows on either side of the door as her kittens. She meows all night and day. There’s no urgency to it, just a flat mrooww - mrroww - mrroww.

We are losing our damned minds. Any advice is appreciated.