This is an irregular feature - both in frequency and oddness - dedicated to a word I came across that I have never previously used.
idioglossia (ih-dee-oh-GLAH-see-uh) n. a secret language between twins; a form of secret speech or language, especially those invented by children; a psychological condition in which speech is so distorted as to be unintelligible.
Derived from the Greek root words idios ("distinct") and glōssa ("tongue"), idioglossia most frequently refers to the phenomenon known colloquially as "twin talk". These types of speech are also known as autonomous languages or cryptophasia.
Studies have demonstrated that some forms of idioglossia exist in up to 40% of all twins, but typically disappear within the first two years of life. There have also been documented cases where children - not necessarily twins - develop such languages when there is an absence of adult models from which to learn.
The 1994 film Nell, which starred Jodie Foster, is a fictional representation of a young woman who was raised by her mother in an isolated cabin, and whose unusual language ("Nellish") reflects the early years that she spent with a paralyzed mother with speech defects.
I do not know, however, if the grunted language among users of weight equipment qualifies as idioglossia, but one might make a convincing case for this.