For some reason it shows up in the audio indicator bar at the top of FL's GUI, but the master track of the mixer doesn't pick it up. It's a simple waveform but, and I can't stress this enough, sufficiently loud to be absolutely fucking ear-piercing. Occasionally, when stoping the playback, FL Studio will suddenly output an extremely loud sound, that will not stop unless I close the software or shut down my audio device.
I've been dealing with an very annoying issue for some time.