What you are hearing is interference coming from local radio stations.
Now a normal audio amplifier will not pass RF from a radio station but what is happening is the first stage of your guitar amplifier is a non-linear device, probably a PN junction of a transistor. It is acting as a detector and you can faintly make out some music and voices coming from a local station or stations.
Now a normal audio amplifier will not pass RF from a radio station but what is happening is the first stage of your guitar amplifier is a non-linear device, probably a PN junction of a transistor. It is acting as a detector and you can faintly make out some music and voices coming from a local station or stations.