Rattle belly is a nasty bug seen in new born lambs. It is another name for watery mouth. Basically, when the lambs are born if you poke their tummies they do make a rattley watery squish sound. The other thing you may see is that the lambs appear to be drooling. The first thing I do every morning when we are lambing is just stroke the chin of all the lambs to check them.
In all likelihood a lamb with rattle belly will die, however if you give them plenty of anti biotics they could survive. One thing which intensive indoor lambing sheep farmers use in an oral antibiotic given at birth to all lambs called Spectam and this can be used as a preventative measure.
Rattle belly is not normally a problem in small flocks.