Some of the mp3s I have this noticeable crackling and popping white noise behind all the music, like the sound of a really old phonograph is playing in the background.
Is this due to the mp3's bad rip, codec settings or soft/hardware problems on my computer? Please help I've had this problem for ages and haven't been able to track down the problem through numerous searches =(

Maybe with some more information we could find a solution. Did you encode the MP3's? If you did, what codec? What parameters? What are they of? Kind of try to help us here. Thanks.

When I had this problem it was because my soundcard resampled everything to 48khz

sorry, my fault for not providing any information.
here is an example of the crackling noise that I hear on a lot of my music.
http://rapidshare.de/files/9185790/noise.wav.html
I did not encode this, but it's encoded at 128kbps and is an mp3, which is all i know.

A program like EncSpot might help you to identify a common trait among bad sounding files that isn't shared with the good ones.

Unless it is encoded at 64 kbps or lower, it is not the encoder.

Burn an audio CD out of a few such bad MP3s, having decoded them to WAV with Apollo first, to rule out software issues. If you don't hear anything wrong when playing back the CD on a standalone (or a portable) CD player, then it is your computer's hardware and/or software settings.