A stomach rumbling is a nominal sign indicating that someone is probably hungry.
Communications using signals which are signs, such as my stomach rumbling, may be important but they are NOT examples of language, as we will be defining it later.
Incidentally, if you are not familiar with rumbling stomachs, you should rent the old movie, "The African Queen."
There is an interesting scene at the beginning of the movie which exemplifies this point.
A stomach rumble is a nominal sign because it is an instinctive response arising from internal body processes. I have no control over it.
It is graded in the sense that the more loudly or frequently it rumbles, the hungrier I appear to be to someone who hears it.