On my Revo S baitcaster, the spool is rattling. When I put my thumb on the spool and move it side to side, the spool rattles a considerable amount. Anyone know how to fix it?

Yeah, you have the free spool tension (cast control cap) set to lose...  

The factory ZERO setting is when the CC cap is adjusted so that there is NO side to side play in a free spool.

You can go tighter from that point but should never go loser than that.

Good Luck & Tight Lines!!!!  

Whoops, forgot about that.  :-[

Thanks.

Yeah the best way to set it up is go to a quiet room, and adjust the CC Cap free enough to get a good rotation out of the free spool, and keep adjusting it so you hear no spool shaft hitting (taping) the metallic spool shaft shims. This gives you the factory starting point for max free spool, and the ability to tighten from there.

That is how I set all BC reels that I service before they leave my shop...

Tight Lines!!!