My male jacksons does that too, its like shaking or trembling! he is very tame
and friendly too and sometimes leaps onto my hand and snuggles on my shoulder, so I know it isnt a warning sign to keep away.
I also noticed that my male veiled will emitt a low frequency 'vibration' when I pet him under the chin.
I have read that it's believed (but still not known) that this is a kind of communication within the same species.
As for the trembling jackx, I have no clue !!