hmmm, I think I got some weird static-like noise first time I plugged MP3 player in AUX. Turned ignition off, turned it back on with MP3 player still playing, and it worked then and so far without any problems.
I guess the HU (headUnit) in Beca will be trying to adjust to whatever your particular AUX device output level is. From various audio-topics at legacygt.com I've read that many MP3 players and XM/Sirius devices do not provide adequate line-level output. AFAIK, iPods were listed as ones having lower output levels that needed.
hence the playing with volume knob, I guess...