It could be the "A" joint or it could be one of many other things. Other than a loose weight bolt, it's pretty much going to be a glue failure at some point. It could be anything, and will need a good inspection to be sure. A lot of glues makers use cure harder/stiffer than the wood. It won't flex with the wood as the cue flexes & vibrates during play. A fissure will open in the glue joint to allow the flex. The buzz sound is the crack in that glue opening and closing at the same frequency the cue vibrates at. Aside from a weight bolt or glue breakdown, it's cracked wood. In any case, if you can't find it easily on your own, you'll have to get it looked at, preferably by the maker.