most of times i think its from the fins. i know from windsurfing, if you go realy fast or if your fin is a little destroyed on leading edge, the fin began to “sing”.
the noise you mean comes from turbulents at the fin.
Why does a fin hum? usually it’s because it doesn’t know the lyrics.
Or: Leading edge too sharp. Trailing edge too dull.
I once had a fin that hit a perfect Bb on a bottom turn.
I once had a fin that hummed so bad (back in '65) I could feel a cavitation on a stall or a cutback. It turned out the board had a twist. I had ordered the board (from a well known South Bay maker) with “very very thin rails”. They made the whole board “very very thin” and it twisted in short order.
Most noise comes from the trailing edge. I was once told that if there is a flat spot on the trailing edge that is wider then 1/32 of an inch it will hum. If you take a razor blade and scrap down both sides of the trailing edges in the thick areas you will save yourself a lot of sanding time.
The noise comes from something vibrating at a high frequency. the board itself will not resonate at high frequencies, but a fin will. If the leading edge is too sharp, you can get little vortices, or eddie currents, that flow off the leading edge. The same applies for a trailing edge that is NOT sharp. A bead of resin could also have the same effect. At certain speeds or angles of attack, or combination of the two, the vortices that come off the fin make the fin vibrate at its resonant frequency and thus it gets loud enough to hear.
The beautiful “hissssssssssss” of a long, fast, swooping bottom turn…MMMMMMMMM!
The definitive “smack” from off the lip…AHHHHHHHHH!
The pronounced “thump” of a perfect landing from a good elevator drop…OHHHHHHHHHH!
(excuse me…I think I have to go change)
Then of course there are the bad noises… Oh!..Ummmmm… that of course I NEVER hear.
There’s the “CLONK” of the board hitting you on the head after you’ve been dragged over the falls. UHHH…So I’ve heard.
The “CLACK” of your forehead coming down in the center or you board after it got sucked out from under you, then the wave drives you head first into it.
I…I mean…that’s what I’ve been told anyway.
The “Tump clump clack thonk” of your brand new, custom Mike Barron shaped board getting mangled by some kook on a POS pop out rental.