export to mp3 introduces strange sounds

Hi
I'm using sonar 8.5 producer and I purchased the mp3 encoder from the cakewalk store. Often when i export to mp3 the mp3 file has these subtle but noticable digital sounds. i don't know how to describe the sound but its almost like its in slow motion slightly and sounds like there is a metalic kind of filter over the whole sound. Sometimes though the export works fine. Has anyone experienced this and is there a known way to fix it? should i be trying a different mp3 encoder?

How are you auditioning the MP3s? On your computer, using the same speakers you mixed on, on a portable player, or a car stereo?

I ask because the artifacts may not be in the file itself, but may be aliasing from the playback system.

And what bitrate are you encoding to?