This is to remove Backgroud noise from a sound file (the Shhhhh/Hiss/Humm), not an instrument, vocals, etc.
The program used in this tutorial is Audacity. (click to download)
It's a free, open source software for recording and editing sounds in Linux, Mac OS X, and other operating systems.
I have recorded a song off an analogue radio, as I knew that would produce plenty of background noise
1) First things first, open up the Audio file in question.
2) Next click and drag over a selection of audio where you know it should be JUST SILENCE, but there is "SHHHHHH" instead (In other words, there should be no actual music here at all, just SHHHHH. This would most likely be near the end of the track).
8) Now we are going to Export (save) our new audio file. The proffered setting is WAV, that way you don't lose any quality through compression, plus WAV is compatible with basically everything. The next best setting would be FLAC. It has the same quality as WAV, but takes up less HD space. I'm going to proceed with saving as WAV, as FLAC has less codec support in Default windows programs.
Simply open up the "File" menu and select "Export as WAV..."
9) Now select the location where you would like your file to be kept, and also give it a name...
This does not completely remove the noise from the audio, it just "masks" it from being heard as best as possible.