Hi,
This is a long story but it goes somewhere possibly relevant.
Some years ago, I took in a cat that my parents had owned for a long time. When she lived with them she was very irritable, intractable, but only with people; they had several other cats, and she got along fine with them.
She was a great-granddaughter of a cat I'd owned as a child, who had one litter of kittens (oops) - we found good homes for all of them. So none of us wanted to give up on her, but my parents were growing old and really couldn't deal with her. They couldn't pick her up, take her to the vet, couldn't even pet her without being shredded. She was just impossible, and getting worse.
First night she stayed with me I heard a very strange cry at about 2 a.m. Since I had several cats, I got up and went looking, and I found her proudly sitting in front of a catnip mouse toy. Very proud. Sitting tall and all fluffy and excited. Eyes glowing. So I praised her and took the risk of petting her, and she was very receptive to being petted. (unusual to say the least).
After that, I heard the same "announcement" from her each night, always after I went to bed. I got up and praised her and petted her the next few nights, then after that I'd just call out praise to her without getting out of bed, always using her name so she would know I was responding to her call.
Before long she was sufficiently at ease to engage in her hunting play during daylight hours and where I could see her, and always she would go through the play of stalking, hunting, then at the end she would announce her "kill", and I always praised her. Not long after that, she began to bring the mouse to me and set it at my feet before making her "announcement".
You will not be surprised that she also became calmer, much more approachable, and finally reached the point where I could lift her and kiss her on the top of her head. Amazing?
But the explanation, I think, is simple. Her great-grandmother was a formidable mouser, and at that time the neighborhood was rural so it was safe for cats to go out. Development brought too much traffic for that, so little fuzzy buzzsaw could never go out, and my parents simply didn't think to give their cats toys.
She was just about frustrated to death, I think, and as soon as she had a way to relieve the pressure of this frustrated instinct, she began to "heal". My other cats never had a problem with her, and she became a very affectionate, true pet. She would greet me each evening when I came home and purr so loudly it could be heard throughout my apartment.
Her hunting announcements sounded very much like the call you describe. "rrr-RROWW? rrr-RROWW?" very bell-toned and clearly meant to carry some distance.
What truly amazes me is how strongly she wanted not only to "hunt" but to be praised and appreciated for her successful "hunting". No wonder she was frustrated with my parents. They wouldn't allow her to provide for them, and didn't give her a play outlet for this, although they had no way of knowing that is what she needed; it wasn't their fault.
Maybe your cats are similarly announcing that they have protected you from spiders or centipedes in the night, or are "bringing home the groceries" in the shape of catnip toys, too.
:)