Please forgive any mistakes in advance. Bhai Kulbir Singh ji can shed more light on this.
For years upon years I came across non-AKJ Gursikhs who would question and even mock the way Singhs from the Jatha do saas giraas simran. Few of them even said that they make sound like pigeons (referring to Gurrrr Gurrrr). I never knew what exactly to say, until at a recent rensabhai I got my answer.
At a certain stage, one's simran speed becomes faster than normal. The thirst of Naam is such that the more you japp it, the more it increases, unlike water where you drink a few sips and you are satisfied. That is where the sound Gurrrr Gurrrr originates from, the desperation of the person doing simran, its what you would say in Punjabi "pyaar vich tadafna". I don't like when people are clearly imitating other Gurmukhs and make such sounds. You can easily make out when someone is doing it out of rass, or merely pretending. Next about the pigeon sound. During such a simran moment, it is very much possible for one to feel bairaag, which directly affects your throat and thereby your voice. The voice becomes high pitched. Now combine the high speed of simran done by a thirsty soul who is also in bairaag, you get what those people call pigeon sound. There are youngsters who imitate such sounds and it is so obvious as you don't see them in rass (or maybe I am blind to it).
This is just my understanding, I could be totally wrong. Please forgive and correct me wherever I am wrong.