yes, the noises can be quite alarming.
I recall that on one of the occasions we went to a day centre - before they banned Jan for being disruptive - a carer wheeled a lady in and she was making all sorts of weird sounds. I recall being a bit annoyed at the disturbance at the time - it was at Christmas and they had some singers in.
I guess it came back to haunt me, because Jan makes the identical noises now.
Different person, different noise is often the case.
What I have learned to remember is that for each of us, the brain controls everything we do, and that includes speech. Because the dementias hit each person differently [and that is also day by day], speech is affected differently too, and because their cognitive ability and stage in the illness varies, the combination of all these means that speech is a major challenge to them.
When they are saying "hello" it may sound really threatening because they can't apply the nuances of voice we expect.
Jan's voice has dropped a couple of octaves in the past three years and is now mostly a deep monotone - except in periods of a bit of clarity, when the intonations all drop back in and she sounds just a tad more as she once did.
At her home, there is one man - another early onset person [they have quite a few there] - who lurches up to you sounding very threatening. After a while of avoiding him, one day he got me in a corner, so I just said "Hello [his name], how are you today?". He tends to look down most of the time because he has the common problem of being unsteady on his feet, and being unsure of his walking. As soon as I spoke normally to him, he looked me in the eye and smiled, muttered something else to me [it could have been "I'm ok" but was indecipherable], then he turned and walked away. I think he was just being polite, but the dementia makes this seem threatening.
Since then I have always said hello to him and the others each day. Sometimes he comes to the room where Jan and I crawl on the mattress, and looks in. I say to him "It's ok, I'm just looking after Jan"; he smiles, then walks off. I think he is looking after her, in his own way.
Message: don't be too put off by the noises. The people are generally just trying to communicate. It does take geting used to, it is alarming and a bit distressing at first, but they are still people locked up inside a body controlled by dementia.