I picked up a used Longboard with a glassed in fin recently. The few times Ive taken it out I've noticed a humming vibration/ noise at slower speeds. I had the same problem once with a different board and was told it could b an imperfection with the fin.
Make sure your fin is clean of wax.
Check the leading and trailing edges of the fin, and sand out any nicks (if it's a fiberglass fin).
Check the edges of the patches that run up the side of the fin... be sure they're faired well.
Dull the trailing edge of the fin, particularly at the tip.
Most of the time, if it was an inexperienced glasser, the hotcoat has ripples or other irregularities, and a careful sanding solves the problem.
LBcrew is right on- but if the above is not the issue- it could be a miss alignment of the fin- usually the cant and the toe-in of the side fins. Sometimes you can see it with your eyes- sometimes you gotta measure it. I have repaired fins on boards before and when its not right, usually you get the hummmm.
The fin is in excellent shape and it's a Bing so I don't think that its an issue with the glasser.
Bing is just a label, not a guarantee in quality. Matt has plenty of ghost shapers learning under him.
Not thread related, but are you talking about a green Levitator with a red fin? If so, my custom 9'6" Elevator shipped to NC with your Levitator. Just a cool note, considering only 4 boards were shipped on that order.
If I do find any irregularities, what grit sandpaper should I use?
I usually sand out any nicks or bumps with 100 grit to get the meat of the work done, then progress through 150, 220, 320, 400 wet. If I'm just dulling the edges I start with 220 or so, doing a little at a time, until it stops.