How does water in mains pipe make a noise like water running?

[Neil23]

In our semi-detached house - and also other houses 50 yards away recently had an issue whereby we could hear the mains water pipe sounding like water was running somewhere - not the actual trickling of water - but the sound of like a high pitch hum etc 24/7.

The water board inspector came out and said there is air in the mains pipe somewhere - about 2 weeks later our mains cold tap 'burped' out a huge dose of air and the noise has stopped...
My question is - how can air in a mains pipe make the pipe sound like there is water running somewhere - when in fact the water isn't running? Anyone?
PS the problem appears to be fixed but I am curious to know what makes the noise.

 

[johnduffell]

The water was probably running in the mains pipe in the street and the noise was conducting through to the communication pipes to each house.