The Pregnant's Swarmers could have been an infant's rotting corpse that has been morphed after the Pregnant has been infected. They could also be made of random bits of flesh of pieces of the Corruption that got close to the Marker's signal and reanimated, the way these pieces could be made is them being torn off Necromorphs or survivors/corpses. If you look closely at some screenshots, you can see that they appear to be made out of human hands, albeit the Necromorph infection has made the fingers longer and thinner than an average hand, as well as the entire hand being noticeably smaller in proportion. During Dead Space 2, Swarmers are seen leaving the corpse of Sally Hennen further suggesting this. It is unknown what forms a Swarmer, though it is possible that they are composed of small bones such as from the fingers and connecting tissue or cartilage.

It is possible, however, that Swarmers create these sounds by rubbing certain 'body' parts together, much like real-world grasshoppers and cicadas. It is unknown how they do this, as they seemingly have no way to emit sounds. Swarmers make high-pitched screeching noises and barks when encountered. When Swarmers have no target in sight, they stay in place and flail their bodies around. Swarmers often reside inside Pregnants and only come out when the latter's stomach is ruptured, but are also encountered from time to time in independent swarms or, bizarrely, inside boxes, where they jump out onto unsuspecting victims.

Dead Space Pregnant + Swarmer Necromorph Sounds HD

Dead space 3 frozen necromorphs