Hitting railings and metal side of a mysterious tank-like machine

In this derelict building is a very surreal object, a cubic metal container seemingly built on top of what was once a military tank - the gun barrel is still there but sawn short. It is totally bizarre, faintly comic, sight, in a white tiled room half open to the sky, decorated with graffiti. There are metal ladders attached for climbing in and out. If hit with the hand all these railing, rungs and sufaces sound at slightly different pitches and with a deep reverberance. At two points my mobile phone interferes with the recording. Who or what is trying to communicate? A pair of swallows have built a nest in the far corner of the room and are flying around soundlessly, worried at my presence.