Babbling
Studies show that manual babbling indeed exists and it occurs in deaf infants who are exposed to sign language from birth. Even if a baby is hearing and is exposed to sign language at an early age, the baby would manually babble. Manual babbling can be found in hearing babies of deaf Ameslan parents who expose the language, ASL, to them. The similarities between manual and vocal babbling suggest evidence for the ontogeny of language, spoken or signed. Longitudinal studies also show the similarities of developmental patterns of language and communication in deaf and hearing --
from manual/vocal babbling
to manual/vocal gesture (prelinguistic communication)
to early lexical development to full-fledged language.
The development of language in the brain occurs regardless of which language is and which modality is (manually speaking, vocally speaking, or writing).
References and suggested readings
Petitto, Laura Ann and Marentette, Paula. "Babbling in the Manual Mode: Evidence for the Ontogeny of Language." Science Reports. Vol 251, 22 March 1991. Page 1493-1496.
Petitto, Dr. Laura Ann. Untitled. Montreal: McGill University media. October 28, 1997.