There are urban legends, and there are country tales, all invented to ensnare the mentally challenged.

Most of the old folkishness about fiddles appears to come from the tradition that "fiddle music" was somehow "evil" and that anyone who played the fiddle was sort of disreputable. (Some things never change.)

The accusation that "fiddling" was somehow connected with "devil music," made by superstitious non-fiddlers, naturally called for the fiddlers to respond with "magic" amulets and rituals. The difficult question is whether a given "magic" was because the fiddler believed it did something, or whether the fiddler did it as a put-on of those who denigrated his art.

My explanation of the rattler in the fiddle goes:

Every kid that's ever seen a rattlesnake probably killed at least one and took the rattle as a souvenir. The rattles dry out and get fragile, and there's no place safer to put one than inside your fiddle. People will ask "why," so you make up some B.S..

People who believe your B.S. will go get their own rattle.

John