-> What I believe you are hearing is essentially an acoustically generated comb 
-> filter.  Sound waves traversing the clouds encounter several dramatic 
-> temperature gradients.  These are randomly placed of course, due to the 
-> ever-shifting nature of the clouds.  The nature of the engine noise is 
-> basically pink noise.  As the wavefronts of the pink noise traverse these 
-> thermal shifts they are delayed by a very few milliseconds (or possibly seve 
-> hundred microseconds.)  As these delayed wavefronts meet randomly and 
-> "recombine", the result is the "phasing" (or more
likely "phlanging") effect 
-> you hear.  I suspect that the snow clouds exhibit more drastic temperature 
-> gradients than others. 
 
-> Make sense?  Or am I off in left field somewhere? 
  
Makes sense, but I'm surprised that the temperature gradients are 
*that* much greater in snow clouds than others. I've never heard 
anything similar in thunder-clouds in summertime, for example. 
  
                                   dow 
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