I do not know if an anyone has done this before but it works!
It works by seeing in a Silverfish has gone in a stone block.
Then it makes the block stone and summons a Silverfish.
More Silverfish the higher chance of getting a polse.
Make shore it has a slow clock or it will not work!
Make shore you use xcord, ycord, and zcord are the stone's coordinates.
+1 means the coordinates + 1;
Please tell me if you use it :).
/setblock xcord ycord xcord stone
/summon Silverfish xcord ycord+1 xcord
Use it to spawn mobs.
I am the one hew posted it.