Knocking the puck of used, compressed coffee grounds out of the portafilter into the knock box, after pouring a double shot of espresso. Usually a couple of taps are enough; this puck must have been a little messier than usual. Derived from file0427.wav (shortened), which was recorded 6/16/2009 using an MAudio MicroTrack 24/96 digital audio recorder and Sony ECM-MS907 stereo microphone. A photo at http://www.flickr.com/photo... shows the setup. Also posted to wikiGong.com.