The Language Log Dinosaur (as I like to think of him) makes the interesting point that 'no natural language uses the "blowing a raspberry" fart noise sound as a phoneme.' It's worth pondering why not -- for although it may be a noise too laborious to form (poking out tongue, coordinating the labial, getting the breathing right) to be able to fit easily into the flow of normal discourse, it stands very well on its own. Which is to say: 'It is un-["blowing a raspberry" fart noise sound]-believable' doesn't flow well enough to be selected for communication (unlike 'un-fucking-believable'), nevertheless 'My opinion of that? ["blowing a raspberry" fart noise sound]. That's my opinion' would be a completely ordinary piece of expression.
So, ["blowing a raspberry" fart noise sound] is a part of speech. But wait, given that it occurs in speech (like 'I' and 'Mm!'), in what way is this not a phoneme?