Picked up a used board with Alpha Controller quad fins. When the board picks up speed, I get a fairly loud humming sound. Have to say, pretty weird. Anybody know what is going on? Thanks

Try lightly sanding the trailing edge of the fins sometimes fixes it.

Also ensure you have no loose fins / fin boxes

I ran the Alpha Controller quads on my Super Frank and they hummed loudly as well.
It's just the design of the fin causing it id say.... its nothing to worry about.
In fact the hum will just alert any lowlife drop-ins that you're coming in hot up there clacker !!

So after checking out the type of fins you have, I came across this. Not sure but it looks like they're a moulded fin and probably need a sanding down particularly of the trailing edge.

Humming ==> Donaldson!

colas.nahaboo.net/Surf/TheDonaldsonTrailingEdge

or: www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/The-definitive-solution-to-fins-and-foils-whistling-humming--The--Donaldson-trailing-edge-

Thanks all for the replies. Colas, that video was awesome thanks. I sanded down the trailing edges today and will see next time I am on the water. The fins have grooves on the trailing edge. I decided to just take them down to close to smooth. Interestingly, when I sanded the rear fins the first few strokes sounded exactly like the humming sound. Just a hunch but after sanding the grooves down I wonder if they were the cause of the humming.

Select to expand quote

castawaycove said..
Just a hunch but after sanding the grooves down I wonder if they were the cause of the humming.

It could be, if the grooves creates a trailing edge wide enough for the trailing vortexes (vortices?) to oscillate. But I have no real experience.

Grooves on the trailing edge are worth smoothing out to avoid fin cuts to yourself and the leash, they can act as a serrated blade.