I saw your other thread in Education.
I think this is one you need to take up with the boarding house staff. You can't sort it remotely, this bit of pastoral care lies with them while she is in their care.
It can't be just the noise if she is now not sleeping at home. And at 5am I really doubt it would be the noise anyway. With insomnia it is quite common I think for the sufferer to feel like they haven't slept a wink while others report they looked like they slept soundly. I guess the truth probably lies somewhere in the middle - she is getting more sleep than she thinks (though I'm sure it would be tricky to convince her of this) but possibly less than the boarding staff report.
I think you need to go back to them with the new info you've gathered over half term, ie that she is waking early now, any other evidence of wakings, what she's said. Repeat that DD feels like she is very short of sleep. Them reporting that she is asleep at X time doesn't change the fact that she is feeling underslept and tired. You had a bashing on the other thread but I think there is a grain of truth that there might be anxiety behind this, and if the boarding staff aren't cognisant of that and prepared to take steps to reassure her, then are they the right people to be in charge of her pastoral care?
She's autistic and there's been a huge change in her life. I do think it's normal for any child going to boarding school to react to this huge change, and it's still early days. I remember exactly who was most homesick in my first year at boarding school and I'm sure it lasted months, not weeks. With autism in the mix the change is likely to be that much harder for her. It's a huge adjustment, it will take time. I'm sure you made that very difficult decision only in her best interests, but it's still OK for her to have a reaction to being taken away from home, and you can expect a few bumps in the road while she settles.
Sorry for waffling on. I would tend to work on the basis that it's a reaction to the change, ride it out and reassure. However it's perhaps a measure of the pastoral care there whether the staff seek to help (if only to address her anxiety) or continue to say there is no problem.