Freeyeow! In this day and age, there's no excuse not to be using a variable bit rate encoder. I mean, I don't use .mp3 anymore unless I absolutely have to, but if I must I demand VBR. That's the only way to get rid of the swishy sound .mp3 always imparts to rapid high-freq percussion without ridiculous bitrates (like 256 kbps or higher, sheesh, thankyou, no).
Better still to just use .ogg. There are a number of good portable players that support it out of the box. For those that don't, there's the Rockbox mod. I've found an .ogg with an average bitrate of around 130 to 140 kbps is more than good enough to fool my ears.