I have found that when I swallow air until my belly is overly full in order for me to hit my 20-30 second long burps that some of that air always ends up being forced down into my body while most of it comes back up as the huge burps that I was aiming for. So a few times now I have set up my laptop and recorded the farts when they were ready to be let loose! 
https://youtube.com/playlist?list=PL_0sDQRVabfScf6ZNFfQ4X5ZTvqB8BEBs
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