Howling sound when flying

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.