Emu XL-7 & MP-7 User's Group

Playing XL-7 preset 21:2 (bas:Sonic), there's a 'thud' at the start of the sound which however is only present if the sound hasn't been played for half a second or so. That is, when playing a continous melody, the 'thud' is only heard at the beginning of the melody.

It seems the 'thud' is present already in the basic waveform (instrument) used, and somehow it sounds like the START parameter is modulated somehow. For instance, one way I guess would be if the auxilliary envelope were set up in such a way that it would advance the START parameter (due to a long release) if a note were repeated. However, I can't find a patch cord which actually does this, and the aux envelope doesn't look like it's doing anything in this patch.

Has anyone managed to figure out why this patch sounds the way it does?