It happens in hosts where MIDI data of any sort is sent to FX. EnergyXT, for instance, does this by default to all it's FX and allows MIDI to reach them.
If the MIDI track has any data at all (notes, CC changes, etc.) and that ends up reaching Parametric EQ2, EQUO, Delay2 etc. then you get a grainy corruption of the audio signal.
------------- example --------------
A single note playing for 4 bars. Lots of cc's in the 2nd and 4th bars and a couple in the 3rd to show single clicks. The FX is IL ParametricEQ 2, just on a standard insert.
Here's the accompanying WAV file to show how it sounds:
http://www.ccir.ed.ac.uk/~nanderso/juicepack.wav

