At least in the US, air conditioning / heating tends to be on the loud side, since the AC unit is mounted directly inside the room. Often times I resort to turning it off for the night to make sure I can get some quiet sleep. This is an issue in both cheap and middle class hotels - not sure if 5-star places have the same noise problems.
What's the reason behind this frequent problem? Is there a general expectation that such noise would not be an issue for most guests?