That's the typical sound they make to express disapproval. They use that sound to warn of a potential bite and also when in the process of bickering with a mate.
You will have to see the context in which he makes that specific sound and take that in consideration. The situation you have described (petting/wanting to have a closer interaction), it means that he wants you to back off, respect his space and to let him be alone for a while.
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RIP sweet Tito (Summer 2008 - January 17th 2013).
You are missed and never will be forgotten.
aluz has answered your question regarding the particular sound Cosmo was making that you were concerned about.
I have now merged seven threads you've created regarding Cosmo's behaviors, including different sounds he makes.
If you have additional questions and are unable to find the answers in the Budgie Articles and/or Stickies, please post the question(s) in this thread rather than starting a new one.
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