Don't confuse the percussion feature with "key-click", which is the sound of the contacts under each key connecting when the key is pressed.  It's almost a "static-y" sound.  It was a part of the organ that Hammond himself considered a fault and the engineers tried all sorts of electronic solutions to mask it.  A lot of the the jazz guys modified their organs so they would have more of that key-click. 
 
But I know what you're saying about Groove; he sometimes used the 2nd harmonic percussion and it was very pronounced.  Jimmy McGriff modified his B3 somehow to make the percussion really thick and, for lack of a better term, nasty (and get more key-click, too).  His tone on those Solid State LPs is ridiculous. 
 
 
Ah, thanks Jim.  
 
I remember talking to an organ salesman in the early eighties about the Hammond B3000 and he was saying that Hammond had artificially put that click into the solid state version because organists liked it in the B3. (Then he sat down and played "Misty" with a damn fine groove!) 
 
MG