Playback of mp3 tracks

[Tiber]

I recently transferred some FLAC and WAV music files to my Rockbox'd iHP-140. Prior to doing so I converted the files to mp3 to save space.

(Some of the files were live concerts and if they are listened to as WAV there are annoying little joins evident between tracks, something that does not happen when listening to them in mp3 format).

However, I have noticed that when listening to these transferred files there are loud noises, akin to bursts of static or electronic noise, at the beginnings - and only the beginnings - of some but not all, tracks.

These noises are only heard when listening to my mp3 player through headphones or my hi-fi system, if I connect the mp3 to my PC they are not heard.

Is there something obvious I have overlooked in the transfer process or should I just re-convert them? I am not bothered about transferring them to CD and only wish to listen to them on my mp3 without bursts of unwanted noise or audible joins.  

Any advice would be welcome.