It's pretty common to see a rattlesnake rattle put inside instruments used in bluegrass and old-time music. I believe that Bill Monroe had one in his famous Loar mandolin and you see them in a lot of old fiddles. I've never seen nor heard of one in a bass and that's a pretty good sized rattle (and corresponding snake), appropriate to the size of the instrument. There are various theories as to why, one being that the snake essence will keep mice and other vermin from nesting inside your fiddle or it helps capture some mojo associate with poisonous reptiles. My personal belief is that they're put in there because we've always put them in there. BG and old-time musicians are nothing if not traditional.
As long as the rest of the snake is not included I'm OK with it.