The metal pole hanging from the bottom of the gate by the camp site sounded a note every time the gate was opened. The pole was about 1.5m long. The longest stationary wave possible has a node in the middle and antinodes at each end. That adds up to half a wavelength, so the wavelength would be 3m. Assume speed of sound in air is 330m/s, then f = 330/3 = 110Hz. That would be a low A so quite feasible.
