In this National Geographic segment, the narrator explains how sometimes young kids need to help cows lactate by blowing into their butts. The act is a form of stimulation for the cow and if done correctly will allow the cow to secrete milk! [via]

I believe that sound you hear is the cow farting back the air that the kid is blowing into its anus. I cannot begin to imagine how these kids must feel waking up every morning for this job.