As I understand it, the popping was, for the most part, always there, but the baffles made it so you just didn't hear it. Without those there, you're simply hearing that which has for the most part always been there, but you just didn't know it. As such, it is normal...but if you don't care for it, you can try one of the remedies mentioned above. I did the marbling, and had less success than I've read others have had, but it's quick and easy and worth a shot.
( Used to have: )
'08 Vulcan 900 Custom:
*Passion Red *Mustang One-Piece Vintage Seat
*Hard-Krome 2.5" Strippers with Smartpartz Hard Kore Baffles
*Cobra Short Sissybar, Pad & Insert *Leatherworks 312 Chopper Bag *Small/Med. Windshield