It's the same my dad is doing, sing and whistle.
I know that this may sound ridiculous but it is the way they do it. I bet anyone that if they would see and hear they not only would be annoyed but they would be frustrated and even scared. When my dad does it he sounds like someone is about to totally lose it, sometimes it is scary, especially when he does both things at the same time. At first my mom was annoyed by him then she started imitating him and now she does it on purpose because she knows that it is driving me crazy, she is now coming in the same room I am to do it where I am, yesterday she did it over and over for several hours.