Boiler humming/heat exchanger humming

An acoustic phenomenon that occurs, among other places, in industrial shell-and-tube heat exchangers. Uniform vortex shedding, caused by the flow over the tube bundles, resonates with an acoustic room mode perpendicular to the flow direction and the tube axis, forming an energetically favourable, feedback-coupled and thus self-sustaining oscillating system. This can manifest itself in pronounced structural vibrations and/or in clearly audible tonal noises.