Hi,
I'd suggest to switch to mpg123 as default decoder for mp3 files. The libmad decoder produces some low level noise, that can be heard (especially with good headphones) in silent passages of a track (or between two tracks). It's also easy to reproduce if you activate software mixer and turn down mpd volume.
Steps to reproduce the noise with libmad:
1. turn down mpd volume (software mixer)
2. turn up volume of your amp
3. play a mp3
With mpg123 or other file formats (FLAC, Vorbis) there is absolutely no noise. I've tested this on runeaudio and also on my PC (with two different output devices).
Also mpg123 is still activley developed while libmad's last version dates to 2004.