I know this post is a bit dated, so I'm posting for any future searchers. I had a similar scent coming from a kombucha I initially made when I received my scobys. I made a batch of green and black tea, one gallon each. They were created using the same amounts of sugar, water, and starter. I fermented them to an acidity of roughly pH 3 or 4. Each scoby was pretty much the same when I received them, but I noticed the black tea scoby created a baby about a half an inch thick, whereas the green tea created a baby that was only about a millimeter thick. The black tea kombucha had a pleasant scent, but the "farty" scent mentioned in the previous posts was definitely present when I opened the bottles I was storing the green tea kombucha in. Although the scent was somewhat bad initially, when allowed to breathe for a minute or two, it went away. I have noticed this scent in organic milk that comes in cardboard cartons, as well, but when I contacted the company regarding it, they said it was normal and from "good bacteria" in the milk. I removed the green tea scoby that created the bad scented kombucha, and it was fine and made a great batch of black tea kombucha with good growth on the new baby that developed. If you're having issues with how long you should brew your kombucha, buy some pH strips. Be sure they have the 1-5 range included (many start at 4 or 5). When mine reach roughly 3-4, I find the flavor is best and the batches are done. This acidity seems to work great when using it for starter on the next batch. I have read that it is this acidity which keeps the harmful bacteria away while the good bacteria and yeast in the scoby develop.