The Australian Black Swans are very vocal and use the vocalizations to talk to each other, warn of possible danger or show their displeasure. The swans feed, bathe and sleep day or night, so they can be just as active at night as in the daytime. Now, having said this, if something (i.e., predator) is nosing around their habitat (even though the predator cannot access them), they will still issue a warning. Raccoons can climb and will nose around various structures, so this may be the cause of the warning.

If nosing predators are not the cause of the screaming, then you have two spoiled brats that are expressing their extreme displeasure that they are being kept in an area different from their daytime habitat. If this is the case, they'll get over it. The Regal Swan