I have the Philipps Dreamstation. The doctor can set it for a single pressure or for a range. I started with the single set, but have done much better with the range of 6-12.
As for that annoying whistle, the newest headgear fits better because it has plastic wings that sit comfortably on the back of your head and keep it in place on your head.
I also discovered that the noise goes away completely (!!!) if I make sure the tube goes straight up from my head over the pillow (and then to the side to the machine). There’s a clip on ring that you can buy to attach to your pillow to thread the tube through and keep it above your head. I have not needed to add that yet. The tube is long enough for me to position it straight up, tuck it under the top corner of the pillow, and leave enough slack that I can change my sleeping position without waking up. Hope this helps!