Vegetative sounds are sounds that have no intent or communicative function behind them. These include burps, grunts, hiccups, coughs, yawns, gurgles, wheezes, and sneezes.
This term is used almost exclusively when talking about infant language development. Vegetative sounds are usually lumped in with reflexive vocalizations, a class that also includes crying, fussing, and some open vowel sounds. These sounds all help the baby recognize cause and effect between internal states, muscle movements, and sound production. This stage of language development should last only for 0-6 weeks before the baby starts adding in cooing and laughing, while also reducing and controlling reflexive sounds a bit more. Even during these first few weeks, vegetative sounds are very much at the low end of the spectrum.
There are cases where adults -- stroke patients, people in a coma or a persistent vegetative state -- may be reduced to making primarily or only vegetative sounds, but in these cases more specific terminology is used, and focus is on which parts of the brain is damaged, not which sounds are being made.