Popping sound during playback through car stereo

I have a 2008 Prius. If I play my Sansa player with any volume while connected to the MP3 Aux port, I get a lot of popping through the speakers.  I do not get any popping when using radio or CD player at high volume.  Any ideas?

My first thought would be to upgrade the firmware on the player.  After that check the cable.