I've posted several alpha release of shoutcast~ for linux, some outputing very bad sound.
Olaf gracefully merge our codes into an official release that you can get @ http://www.akustische-kunst.de/puredata/index.html
the linux version still has a problem : resampling does not work, you have to use 44100 Hz.
if you're still getting crackles, consider removing -rt flag for pd ( if your icecast server runs on the same box ).
It can also be caused by an hardware limitation. i did test it on a Celeron 800Mhz / 128MB, i don't know about minimum hardware requirements.
cheers,
Yves/