Howling sound when flying

On Saturday, October 12, 2019 at 11:42:44 AM UTC-4, Armand Charbonneau wrote:
My Ventus 2c makes a howling sound when flying at 0 flap setting. It is significantly less noticeable immediately after changing the flap setting either positive or negative. Pilots tell me they can hear it when flying overhead or landing. I can hear it seemingly behind me, inside the cockpit. Gear up or down does not seem to make a difference.

The glider was refinished with poly in 2012, so I suspect the wing seals are seven years old. Otherwise the ship in very good condition and soars nicely.

Does anyone know what could be causing this sound? I can replace all the seals this winter, but they appear to be in good condition, except maybe on the tail plane.

It is surprising that this has not been aired before.
When I first flew my PIK 20 E there was a similar loud howl
at zero and neg flap settings. I speculated that this was resonant
vortex shedding from the top surface; the flap had an internal fabric seal
but this will not stop oscillatory flow.
The Aerodynamics prof at my alma mater replied that I was probably right;
resonant vortex shedding on the BAE 146 stol transport was close to
ending sales due to passenger complaints; when CU dept eng. found a solution.