Don't know the technical reason why, but my external battery does the same thing to the MT... I just try to keep them seperated and there is no issue. Mine causes more of a hum than a crackle though.
on a related topic... don't put your nextel phone in your recording bag while taping... nothing sux more than that nextel "chik chik chik" on a recording right in the nice quiet part of a song. DOH!
I do like that the nextel will make that sound on my car stereo while driving... I can always tell when a call is coming in before the phone rings... which I never hear cause the radio is too loud.