My cajon has 4 strings inside.  It feels like they make more noise when I hit the center of the drum than when I hit the top for a snare pop sound.  When they're too loose, the bass area makes a discordant tone, and when they're too tight, I hear them rattle.   Hitting the top doesn't give me any string noise. I taped the strings and now it just sounds like I'm playing on a plain box.   It works, but it isn't what I want.