Here's the crack, but I'm starting to wonder if it's just the way the drum was made. Can anyone clarify?

That's a seam. It's angled like a "booking" seam used in veneering.

Would it be a good idea to fill in the seam with wood filler anyway? I've used this set once on a gig last week and was told there was something rattling around in the bass drum.

Wood don't rattle, it squeaks and vibrates and buzzes. Loose screws with washers will rattle.

To check the integrity of that seam, press down on it going both sides working from edge to edge. Nothing should move at all, the thin shell may give with play but the seam should not move anyway at all.. If it does it may need to be re-glued. If the seam, is moving,.. it will vibrate and make noise as the note fades.

I think OddBall nailed it perfectly! Sound advice. Check first and, "if it ain't broke..."

Brian

That has to be a seam as was said before. Its just to perfect to be a crack or split. But it wouldn't hurt to put in some filler on the outside chance that it is a split or crack.

I would check the screws.

I filled the seam with filler and will check the screws tonight. Thanks.