I just got a replacement MZ RH1 unit and it does the same thing so maybe this is normal. I'm amazed that nobody else has heard it.
As I said before, when I slide the jog lever to change tracks, just before the track plays there is an audible "pop". Please note this is a split second before the music plays and is a much lower volume. I even hear it with the volume turned down.
I'm not using mp3 but atrac tracks. I've even tried older minidiscs as well as a 1GB HD-MD and the sound is still there.
I'm going to connect it up to my hi fi to see what happens and also try copying a track to another MD machine to see if it's still there.