Why do flying wires hum louder in a sideslip? Page 50 of the Feb. 2021 issue of the EAA's magazine Sport Aviation reports that

Like any good vintage airplane, the Sea Bird likes to fly coordinated,
and the flying wires will sing out loud if the
ball
gets off-center.

Why would the wires hum or sing more when the airstream impinges from one side?
All else being equal, a sideslip converts some of the flow's transverse component to an axial component, from the wire's point of view.  (Like many airplanes, the Sea Bird's rigging is mostly parallel to the wing.)  That should reduce vibration, because axial motion in a wire under tension is hard to provoke: try bowing a violin with the bow nearly parallel to the string.
Here is one reason.
It once was common for suspension wires to be flattened rather than circular in cross-section, to reduce drag. those wires were then aligned with the airflow past the wing and fuselage in cruise. However, any time that airflow was not aligned with the wires, they would present an angle of attack to the slipstream and generate lift, which would pull them out of position, stretch the wires, and twist them to increase the local AoA. Then the wires would shed a vortex, stall (killing the lift), untwist, snap back, and then get pulled out of position again, etc., etc. and the result was the wires would &quot;sing&quot;.
You can think of this as a special case of aerodynamic flutter.