When i write songs, i tend to write melodies, harmonies, rhythms, chord sequences and all those things which you associate with songs and the things which make them recgoniseable. However, when i listen to a good professionally made song closely i am inevitably shocked by how little of the song is actually made up by the "music" and how much of the sound is generated by background "noise". Often this is just synths and things playing actual notes, but a small but significant proportion of the time it doesn't seem to be notes at all!

How does one go about writing this background noise/putting into a song? I've tried various things and they either come out sounding like some sort of harmony line or some sort of thick incomprehensible rubbish.

examples are hard to pick out, for precisely the reason you don't notice it much of the time, the middle section of "whole lotta love" is kinda what i'm on about but i want to incorporate it much more subtly in the way sigur ros, porcupine tree and sOuth do to get their "epic" sounds.

so any tips? do many of you actually do this when recording? is it normally done by producers? (something which i will probably never have)
The only 6 words that can make you a better guitarist:

Learn theory
Practice better
Practice more
tips is all about experimenting, try think outside the box, work with Pad synths, ambiance etc

they add it in when recording to thicken the sound and generally all the time makes the song more interesting as you would not hear everything the first or second time round

check Set fire to the hive - Karnivool or Cote by the same band for some good added sounds